You mean ones that are actually engineered for kids, right? Because for kids from tween and up, and cheap tablet with a kid-friendly software setup will suffice, and you can make and market those yourself fairly easily (ie. pick up quality unbranded tablets off of DHG or Alibaba, handle the software on your end, resell at a profit.)
Actual "kid engineered" tablets like the Fisher Price iXL, the uDraw, etc. will tend to be available from any electronics supplier or supplier who specialises in kid brands like FP, VTech etc.
Some good inspiration for the specific products for which you might look are Link hidden: Login to view
and Link hidden: Login to view
, two good review and comparison articles from zdNet and Consumer Reports. I don't know if the Nabi is in either, but I highly recommend that one -- it is an impressive piece of work, and very affordable (unfortunately, sniffing a little all i can find for a wholesale source is Nabi themselves.)