The new tablets (the HD and HD+) have a profile system very similar to the one found in Windows and MacOS. They allow you to set up multiple profiles on the same device, and each profile can customize their home screen and options. You can also lock down sections independently in each profile, so you can disable purchasing or even the shop altogether on your child's profile while still making it accessable on yours.
So if I was in your boat I'd get a NOOK HD (the 7 inch model).
The nook color also has the functionality to password protect purchases, and you should be able to reload already purchased apps that have been deleted.
Where do I go to set the password? As far as reloading the apps I purchased, I can't find them anywhere. I have logged on to my account online and do not see them in archives either. Where would they be?
There are very good replies in this thread. I'll add my thoughts.
1) Nook HD ships with Parental Controls.
As another reader said - buying $134 Nook Tablets from eBay might be a better option - but they don't have parental controls.
2) Nook Tablet and Color and HD all come with the option to turn on 'Require Password for Purchase'.
3) I don't think there's any way to prevent 'deletion' of apps and books.
4) Nook Tablet is pretty sturdy. Nook HD will be lighter but should be sturdy too. B&N does make good quality hardware.
I think it might be best to first get a Nook HD or HD+ for yourself and try out the Profiles feature. If that and the Parental controls work for you, then you can get a Nook HD for your kids and let them share.
If that doesn't, then something else (like two Nook Tablets bought at eBay deal prices - from B&N official store there) would be a good solution.
Nook HD profiles feature is the best I know of amongst Tablets for Parental Control. Kindle Fire HD has something but it didn't seem very convincing.