There is no boarding card check airside at T3, there is when you get to Flight Connections at T5. [In fact if you arrive at T3 you can exit via the UK Border at T5 and re-enter, not that you would need to do so]. However there are also staffed check-in / customer service desks airside on the north side of the Flight Connection area in T5 so they can sort out the kid's card. You can't miss it. Certainly no reason to go landside.