Usage: My son was playing with his food. He dropped it on the ground and the dog ate it and he cried. He was ignorant of the consequences of playing with his food.
In addition to Carlo_R's good suggestion of reckless, you might consider short-sighted
dealing with things that are happening now but failing to consider what will happen in the future: short-sighted plans
Your question specifically asks about being ignorant of consequences. If you also want to consider ignoring consequences you might use
Clearly most of these would not fit your young son, but might suit other circumstances.
A better choice for a child might be "imprudent":marked by a lack of awareness or concern for the consequences of one's acts; rash; unwise.
A child is unaware of consequences; reckless suggests disregard.
Depending on how young the child is, this and other similar adjectives, might have to be used with some tender irony.
Ignorance of consequences equates with a lack of knowledge about what will happen. Ignorance does not equate with recklessness, disregard, carelessness, although an ignorant person may have a surfeit of those habits.
Terms uninformed (“not informed; ignorant”), inexperienced (“Not experienced; lacking knowledge or experience; green”), and ignorant (“Unknowledgeable or uneducated; characterized by ignorance”) perhaps more properly express ignorance of consequences.
Another related word is naive (“Lacking worldly experience, wisdom, or judgement; unsophisticated”). One who is naive (a naif) may often not know the range of possible consequences in any given circumstance.
Previously-mentioned short-sighted (“(figuratively) Unable to see long-term objectives; lacking foresight”) is related to ignorance of consequences in that one who is short-sighted is likely to not look into what the consequences of actions may be. This word is more about ability and attitude than about knowledge of what will happen.