I was having some of the same problems with my soon to be 7 year old. No one has really posted any heights and weights of their kids, so maybe this will help someone. My daughter will be 7 in 2 weeks and is just shy of 48" tall and weighs about 46 lbs with a 20" inseam.
We bought her a Marin Tiny Trail when she was 4 yrs old and she has ridden in to this point. We modified it slightly by removing the BMX handle bars and replacing them with some mountain bike riser bars (the bars it should have come with, btw). The Tiny Trail is a 16" wheel size and although it's the lightest we could find...it's still heavy. Note to bike makers: make a great lightweight bike for little guys! I would pay for it and I think others would too! I see no reason why a little bitty bike should weigh more than my full size mountain bike.
We recently bought a Haro Flightline 20 and I am VERY impressed with this bike...the weight is very decent, the components are terrific for a little bike, the seat is great quality and even the aluminum welds look perfect (not like gooped up toothpaste you see on some). We had the shop fit her on this bike, a Marin Hidden Trail and a Specialized Hot Rock. All the bikes are a bit big for her, but the Haro was the only one she could touch on and that didn't stretch her out too far to hamper turning and normal operation. Side by side, all the bikes looked comparable with only subtle size differences, but the fit,design and quality of the Haro made this a no contest in my mind. I also want to note that the suspension legs in the front are aluminum (huge weight savings over the 16" Marin she's coming off of).
Well done Haro! :thumbsup:
Hope this helps someone else. I never even knew Haro made a little mountain bike until I read this. Great thread...