can you add iTunes card in a kids account

You can redeem an iTunes gift card onto a child's account (it should then be automatically used for his/her purchases), but you need to be logged into that account to do so e.g. if using a computer's iTunes and you are currently logged into your account : Account > Sign Out menu option, that will then change to 'Sign In'

Redeeming a gift card : Redeem iTunes or Apple Music Gift Cards and content codes - Apple Support