For your budding pianist DS girl has she memorized any right hand songs yet? Has she tried any left hand only? I really think it will take a lot of encouragement from you and even a few good happy demonstrations in slow motion to get her playing both hands. But I wouldn't push for it until she is confident with both hands separately. I base this on what I learnt from teaching Jaykob both hands. Consistency, confidence and familiarity along with an expectation that he could do it got us their.
As for your boy..well that's uncharted territory for me

I would suggest note familiarity games but you seem to be doing that. Maybe try having him sing the notes on the screen for the song your are trying to teach so you can play them. Blindfold yourself, he may find it funny enough to participate. At that age being very familiar with the note order in the song is bound to be helpful.
Set small goals and lots of enthusiasm. Regular daily practice in short bursts will get you there.