Originally Posted by Viich

Local buy and sell / craig's list / kijiji / facebook marketplace / bmxmuseum.com classifieds.... where are you milroy ?

Find something race-specific. The difference between a 10yo entry level bike and the highest end carbon fibre race bike is far less than the difference between that and the department-store BMX

Even a vintage race-specific bike in the right size is going to be better.

10yo you're probably looking for an "expert" size, the 6yo you probably want to look for a "junior" - the 18" bikes will be too small unless he's far shorter than average.
An older bike you might want to look for shorter cranks, but other than that, just clean, grease, new chain and go. You may need to look at tires, common that the rear has flat spots on older race bikes from kids skidding them.