To add and subtract, I'd suggest a soroban, the Japanese 4 and 1 bead per row abacus in hand-held size. Those were in wide use until a few decades ago, and can be operated quickly. They're still available on Amazon and Alibaba. Get one where the beads are cone-shaped, not round; those are easier to manipulate. Don't bother with a "reset button"; you can do that with one finger in a second, rattling down the columns.

It's not a very useful skill, but it's not hard to learn. I wouldn't inflict it on kids.

The 5 and 2 bead form is more of a school thing.

I probably still have one, packed away with the slide rule. My HP-11C is on my desk.