I started on a suzuki JR50 on my 5th birthday. I think thats an awesome way to go. By the time i was 7 i liked to get muddy and ride around more than i liked to race or jump so my dad got me the breeze. We looked into getting a RM85 for me when i was 12 but we got karts later instead and raced those for 3 years. I like both bikes and quads but maybe ask the kids which they want? Its really kind of a preference thing i think. With out a track to ride on i get bored on a bike now. The ltz400 i ride is great even in just a field.