[Closed] BMX advice

Don't own one personally, but check out MafiaBikes. You could also look on Ebay and get a pretty decent second hand BMX for your budget too. If your child isn't freakishly tall for a ten year old then a standard twenty inch-ish top tube should do nicely.

How big of a 10 year are we talking?

Standard BMX's are 20inch wheel size with frame size then being preference, most of the bmx brands  (united, we the people, subroas) do offer smaller size wheels with the rest of the bike scaled down for smaller riders, these are typically 16inch wheels or 18inch wheels, although some do a 20inch wheeled bike with a scaled down frame. Best bet is to try and get into a shop and get them sat on the bike, anything with 16,18 or 20inch wheels will do and should give you a good starting point for size.

My newphew is 8 and is just getting to be too big for his 16inch so I will look at ebay/facebook market place/gumtree for an 18inch replacement. reason being the decent ones which will last longer are circa £250-£300 new but second hand go for around £100-£150. IMO its worth looking at spending good money as although they wont get ridden hard at a racetrack or skatepark they will have a hard life with crashes and neglect etc