Two sided board.

9x9 and 3x3.

1. Double-sided boards (economical and less to lose)
2. Three to four boards maximum. Less is more.

4 should be fine at 4 levels each, totaling 16. Make expansion packs.

for this age group I think it will be important to have solutions available. (printed on reverse of board perhaps). You cold use the solutions side with younger kids as a matching/visual recognition exercise and use the other side for kids who are able to start solving on their own. This would extend the life and usability of the game in the household.

1 or 2 double sided boards that can be used over and over again. If need be for older audience a third board.

Just remembered, we had a leapfrog game once with the pages of each letter of the alphabet that you slipped in. That lasted a week. My young kids destroyed the papers, drew on them, whatever but they rarely made it into the game. You were able to print them off the internet but that became a pain and lots of paper. It ended up on the shelf which was a bummer as it was expensive.

Keep the board 'blank', include a book with the setup configuration and key. My and my child's (9 years) sudoko books we do now have 200 puzzles ranging in difficulty from 1 to 5 stars. If you have several levels (considering a range of age groups) and 20-30 at each level, you can recycle on that level until they are ready to move up. They won't do more than 20 probably in one sitting so won't remember them the next time - you can even go out of order if you want to mix it up.

I think 10 of the easiest difficulty level, 20 of the medium level and 20 of the hardest level. Total 50 boards.

Parents can print more from a website or subscribe for a weekly new puzzle by email.

I am thinking between 5 and 10. This allows for possible sales of "expansion packs" at a later time.

I am thinking that the older the child is the greater the number of layouts. For children, 10 and go up from there. I'm sure Cost will have alot to do with this as well