I agree with Daniel. If there’s no reason to supplement, I prefer to give a variety of fermented foods to kids. It’s way better than a supplement anyway in terms of diversity and colony count so don’t feel like you’re not doing a good job by only giving them kefir. That’s already one of the best forms of probiotics there is in terms of diversity and colony count!
And like food, if you did want to venture into giving them a probiotic supplement, I prefer a broad spectrum probiotic like Klaire Labs Therbiotic Complete which comes in powder form for better diversity, and would cycle it on and off. But again, fermented foods still trumps this probiotic if you can get them to consume them regularly.
I personally rotate between giving my kids kefir in smoothies regularly, with sauerkraut as a side to certain meals, water kefir and sometimes homemade kombucha when I brew. But kefir is also my staple at home.