The child radical and the child kanji are exactly the same.
To remember the sounds し and す we'll use these mnemonics:
There is a child standing in front of you. You look closely at the child but something seems wrong. You rub your eyes and when you open them again there is a sheep (し) in a suit (す) in front of you. You yelp, blink, and it's back to being a child. Are... all children really just sheep in suits?!