mini bmx?

How tall is your child? My 7 year old brother is a bit big for his age and his bike has a 20" top tube. It's a little big for him at the moment though. I would look into bikes with 20" top tubes on DansComp.com.

18 inch bmx
Nobody even asked to see tits what's up with that comment it's almost like you want to get them out

Racer or freestyle?

If racer, go to a local shop and see what they recommend/what fits best. A lot of the time getting them on a bike that fits but has room to grow is best, and every kid is different.

If freestyle, see how he looks on a 20 inch wheeled bike, and if he is too small, then go for an 18 inch wheeled bike.

Here is my younger brother and his bike. It is a tiny bit big for him but my parents and I thought a little bit of growing room couldn't hurt. The thing about buying an 18" is when he grows out of it and you want to get him a 20", you will most likely have to get him a whole new bike. Or at least new wheels which is pricey!

Some kid at my local park, prob 7-9 and kinda short rode one in a serious manner and seemed to like it. Personally id go with an 18", 16" if necessary.

My kid brother has a 20 in Diamondback. But it's just a little big for him.