For me State/Territory/Parks quarters are my favorite modern coin. I would say most can be found
in circulation. But it can be hard getting the different mint marks (P and D) in some parts of the country.
But the ones you may not find can still be bought cheap, 50 to 75 cents each.
Also a favorite is
Jefferson nickels. If your lucky you could find most, or all in circulation.
If not, again circulated examples of the hard ones are not to high of price.
It is nice to start sets with kids, that you will have a chance to finish.
So do not start off with the hard ones that have rare high priced coins.
Like Lincoln wheat cents, 1909 S vdb, 1909 S, 1914 D and 1922 no D are
coins that many of us take many years to acquire.
With the quarter or nickel sets, I think you can set a goal, collect as a family,
and complete the sets. This is a good learning experience.
And another benefit of say the
State Quarter, is you can learn a lot.
The history of the states. You can go into, when they became states.
If you do a coin map set, kids can learn where each state is.
Maybe peek their interest to learn more on their own.