Comment by paytonjjones

"Sight singing" is the classic exercise to develop strong relative pitch. There are lots of resources on this — there was even a sight singing class at my college. It might be a little too challenging (and boring) for a young kid though.

If you're willing to give the app a try, I bet it could actually be a pretty solid way to learn relative as well as absolute pitch. Just manually play "Red" before you start to anchor yourself. I've noticed some improvement in my relative pitch just by practicing it with my daughter. I'd be interested to know if anyone ends up using it explicitly for that purpose.