I recommend you go used at first. It's a lot cheaper, so less painful to get into and out of. For the knee board go with a thick one, preferably with hook and similar noses and tails, make tricks like 180s/360s easier. The pointed ones seems to be better in the turns, IMHO.
For the wakeboard get one with the best bindings you can. Start with one your older can grow into soon, same for the bindings.
I made a wakeboard trainer a few years ago for my son and it works great. The one I made for the bigger boards doesn't get much use but the women love it. They guys not so much, too much machismo, so we (the ladies and I) let them entertain us sometimes with how much abuse they can take/who can pull off the biggest fail of the day and the endless excuses (e,g. The sun was in my eyes, the handle is wet and slippery, the board or bindings are to small, to big, I drank to much, not enough, etc.). Speaking if which don't forget the lifevest, the brighter the better.