Kids grow so fast, unless you are making several of them, its a losing battle.
My daughter wore mine, but did not started wading cold water until she wore my stocking foots and some heavy socks. Around 12.
Grandson #1 was almost the same. First trout trip, borrowed a pair of duck hunting waders my boss bought for his 3 sons growning up. By the next few seasons he was into mine with a few pair of socks. Now, at 14, he has passed me up and will be barefoot into my boot foot waders. When he levels out, I will buy him his own.
At age 6, he will not be able to go very deep or handle very fast water. Depending on height and weight. Duck hunting was a different story and no swift water.