My kids have been playing RPGs since they were around 5 years old - it can be a lot of fun, but it's very different than running games for older ages. My wife and I have found a couple of helpful things:
Separate them by age. As you say, 7-year-old girls and 14-year-old boys want different things. For a while, we separated our teenager from her 11-year-old sister. They each played in different groups with their friends. The teenagers ran around fighting stuff, stealing stuff, and generally giving their id a good workout (with proper parental direction). The younger ones talked to people, collected things, and had a weird array of imaginary pets. Everyone was happier.
The younger ones don't care about the rules. Eyes will glaze over if you're flipping through rulebooks to find answers. Wing it. Some older kids will enjoy messing with the rule set, but the game aspect of RPGs is rarely what draws anyone in. They like to pretend that they're doing interesting imaginary things; focus on that.
Pick a different game. I've got nothing at all against D&D, but aside from perhaps the 1980s red box editions, it's not kid-friendly. For our girls, we mostly ran (and still run) games with fewer rules and stronger themes. We had particular success with the d6 version of Star Wars, as well as Toon and the FASERIP Marvel Super Heroes.
