First off, that’s a great gift! Funny yet super wholesome.

Secondly, holy crap you have a lot of cousins!

while you can get a step counter on your phone (including privacy apps like Pedometer on F-droid), I’d go for a dedicated clip on simple counter. There is something about a physical object dedicated to the task that makes a difference to me sticking to it.

Honestly this advice is just as good as the first tip!