My son is young and currently rides an 18" bike. We have a substantial hill on our road. He has ridden this hill, usually having to stop part way up to walk, pretty much every day for the past month. Last week, he finally made it up the hill, on his bike, without stopping. He just needed to practice and build muscle to be able to do it. (Because good golly are kids' bikes heavy.) There's no simple or quick fix for riding hills, you just need to practice.