Okay, kids grow up quickly, but when they're in the age between 5-8, that BMX bike with 20" sized wheels is simply too big for them. They'll lose interest because they can't handle the bike because of its weight and size. There are solutions, like the HARO 116 which is based on the geometry of what the older riders use and looks the part. Visit a HARO dealer near you and get advised.
HARO 116:-Hi-ten 16" freestyle frame