As the Original Poster correctly identifies, the object of reading to improve ones level in a language is severely hampered when there are too many unknown items of vocabulary, or complex or obscure grammatical structures.
The primary reason for cultivating a reading habit if you want to improve your language skills in another language is to acquire a large amount of new vocabulary, and see a wide range of common grammatical structures in a relatively short period of time.
If your reason for reading is to improve your general language level, as opposed to learning how to translate texts for backwards minded school exams, then the key point in the paragraph above, is that you need exposure to a large amount of language in the shortest time possible. What the research in this area shows is that in order to be able to read effectively, you need to be able to understand between 90 - 95% of the vocabulary within a given text. This means that you won't understand one word in every one to two sentences. If you don't understand a greater amount of the lexis than this, you will not be able to read in any kind of natural fashion, your reading will be slow, your comprehension will be bad, and you won't develop any kind of long term reading habit in that language.
When you read in another language, it is a very, very, very bad idea to use a dictionary if you are reading a text where you don't understand about 5% of the vocabulary. You normally pick up vocabulary not because you've been taught it or looked it up in a dictionary but because you've encountered it in contexts in which its meaning has been apparent or intuitively understandable. To give an example from a reading scenario, if you read that a Vergiloop walked across the cafe and lay on the floor next to Bob, you'll have a fairly good idea of what a Vergiloop might be. It's obviously an animal of some sort, not a table, or an idea, a screwdriver or a policeman. Now if the huge Vergiloop looks up at Bob with enormous eyes when Bob is eating his burger, you'll probably guess that it's not a bunny. By the time you read that the Vergiloop is running down the street barking at cars, you will have forgotten that you did not know what a Vergiloop was when you first saw the word. Now the motivating factor that enables you to work out what a Vegiloop might be is that you understand and care about what's happening in the text. If you don't care and don't want to know then you won't pick up this vocabulary. But more importantly, you won't become an avid reader and won't expose yourself to a vast amount of language. So, if you stop twenty times a page to look up any odd word that you don't understand, you will never get into the story or get any feel for the language or be able to enjoy your reading experience.
Of course if you don't understand a word that is cropping up repeatedly, then this is a good sign that you might want to look up that word. But of course, you shouldn't do this if you're far more interested in what you're reading than you are in looking it up. That in itself is a sign that it's not important enough to warrant looking up in the first place.
A lot of well-intentioned people might tell you that it's a good idea to read books for children based on the theory that these books must have simpler language and that they're authentic. If this happens to you, nod and smile but ignore this advice. Children find it hard to read adult books not so much because of the language as because of the subject matter. Abstract ideas, philosophy, social observations and the like are not easy for kids to come to terms with. They also don't know a lot about mortgages, affairs, politics, poverty and many other of the things which afflict us adults on a daily basis. Adult readers of other language texts do not have these particular problems. Sentences with compassion are no more difficult than sentences with griffin - in fact the reverse is probably true!
It's also the case that texts which are easy or difficult for native speakers to read are no guide as to the difficulty that second language learners will have with these same texts. Again the problems for the two groups are not the same. Long sentences and long words are not necessarily a good indication of a text being difficult for adult learners of a second language. Neither is sentence length. Passages with short words and short sentences can be just as difficult as passages with longer ones. The key difficulty for non-native readers is nearly always going to be the range of vocabulary. Therefoe, a Flesch-Kincaid score, for example, will not be a very useful guide as to the difficulty of a text for an adult second language learner.
So, if you wan to develop a reading habit in another language there are some positive things you can do. Firstly, in order to get the right balance of known to unknown lexis, find graded readers which suit your level. Students ofetn try to graded readers at the level above their actual level. This is a very bad idea. If anything, go for the level below your actual level. This will allow you to read comfortably, quickly and enjoyably. This will mean that you can read far more books, and see far more new words and grammatical structures in a shorter period of time. You are also far more likely to continue reading in your new language in the long term. Also, only read things you enjoy reading. If you aren't enjoying a book - change it for a another one. The idea is to enjoy your reading and therefore read a lot. The idea isn't to punish yourself! Graded readers come in all types of different genres, romance, sci-fi, crime, thrillers, historical novels and a wide range of non-fiction too. There really is something for everyone.
If you read things you are really interested in, then you can often read texts that are much more difficult. So if you're a Formula 1 racing fanatic, you will probably be ok reading internet articles or magazines about Formula 1. Your interest in and knowledge of the subject will compensate considerably for your lack of vocabulary here.
In short, if you want to improve your English through reading, only read things you enjoy. Read things that are reasonable easy for you to read. Do try graded readers. Very importantly, resist the dictionary. But most importantly cultivate a reading habit that you enjoy so that you can read a lot, a lot, a lot!