A dictionary is your best friend here...
A young human being below the age of puberty or below the legal age of
The age of majority, in the UK and USA, is 18 (except for a few states in America) courtesy of Wikipedia:
The period during which adolescents reach sexual maturity and become
capable of reproduction.
What does this mean in terms of age? From NHS.uk:
The average age for girls to begin puberty is 11, while for boys the
average age is 12. But there’s no set timetable, so don’t worry if
your child reaches puberty before or after their friends. It’s
completely normal for puberty to begin at any point from the ages of 8
to 14. The process takes about four years overall.
So, child refers to any human being up to at least approximately eight years old, but conventionally up to about 11-12 or depending on who you are talking to and their personal opinions regarding the use of the word, all the way up to eighteen years old (the age of majority).
On the other hand: -
A very young child or baby.
Note the use of the word 'baby', or very young. An infant isn't just a young child, it's a very young child, not much older than a baby.
So there you have it, yes it's perfectly fine to use the phrase infant child.
Infant is modifying child, such that it is clear the child you are speaking of is a baby or a very young child.
Regarding the grammatical construction this is very basic English. The phrase is working the same way chicken soup, works for instance. Chiken is modifying soup, like infant is modifying child.
The first noun is termed a noun adjunct, attributive noun, or noun modifier:
In grammar, a noun adjunct or attributive noun or noun (pre)modifier
is an optional noun that modifies another noun; it is a noun
functioning as a pre-modifier in a noun phrase. For example, in the
phrase "chicken soup" the noun adjunct "chicken" modifies the noun
You can read more about noun modifiers, on the associated Wikipedia page.