my just 7 girl is small for age and justs fits a 20″ wheel bike (specialised hard rock) with seat post cut down so can get right down
her sister is 2 years older and again small for age but is just outgrowing a 20″ bike
so as said above whatever may not last long unless is large for age and some round our way are small giants!
latest trip to shops was quite depressing I’m no bike geek/gear freak but a lot of the kids stuff is very badly bent metal with full suspension bikes in kiddy colours that i could hardly pick up but in the end that may not matter too much if its a bike they really want to ride!
to conclude if you know size scanning classifieds should be worthwhile if they want to go to bike shop and pick something the kid really likes then suggest go along and help by pointing out that the extra £50(+) is really worth it when you look at proper BB, headset, decent forks and brakes etc or that the bike they are looking at is a pile of overpriced scrap that will fall apart in 3 months