I have gone with the high end stuff. On sale, some of is pretty affordable (bought Specialized enduros kids jerseys for ~$15).
They last a surprisingly long time, even as they grow.
For shorts, I have gone Troy Lee, for my youngest, as they were the only ones that had a decent length. Not cheap, but on sale I paid $35 on time, and $40 the next time. They are super tough, and with waist adjusters, easily go up a full size in the waist. They can also wear the baggy short and jersey of the bike.
My girls don’t like a chamois on the mtb, so just use some moisture wicking underpants