"Too hard for little kids"? Really? This new generation managed to get through HG/SS like we managed to get through G/S/C at their age (hell, R/B/Y because, ooh I don't know, not many people had guides and had to learn the game on their own). And I felt the B/W gym leaders were pretty difficult if one wasn't prepared.
A dual-tye gym could be interesting as a form of Yin-Yang-ish sort of way. I suppose technically we already got that with Blue, and some of the gym leaders when you rebattle them (HG/SS anyway) have Pokémon that are a bit different from their normal team. It could give the developers an excuse to make another pair of twin gym leaders to do just that (or hell, triplets--Cilan, Chili and Cress I don't think count, and even then you only fight one of them). I wouldn't know how the mechanics of it would work, whether one type would make up for the other type's weaknesses, or if two types can actually work together or something, but I think it would be a neat inclusion.