'It might be too much given that we know that the region of the brain that IS critically involved in planning is not developed yet'. I think this would be easier to understand if there was a comma after 'much' and also there seems to be an 'is' missing as I have inserted. The first 'It' in the sentence refers to some information/instruction which has been given, and which might be too difficult for the child/young person who is receiving it to understand, because the part of their brain involved in planning is not developed yet.

Paraphrase: It might be too much because of the fact that we know that the region of the brain etc.... Given refers to information already known/'given'.