With kids, I usually aim to get probiotics from foods. Here are some other options you could consider (and might not have given it a try yet). Some have a recipe you can find in the B Better library under the Discount & Resource section
• Beet kvas
• Kombucha
• (Red) Sauerkraut
• Pickles
• Tepache (fermented ‘tea’ from pineapple peels)

I don’t know how old your kids are, but if they are old enough to make the recipe together, it can increase compliance. More important, it can be a lot of fun as well!

 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.

 keep in mind that transient probiotics usually stay within the body for 3-5 days so if you can give a probiotic-rich food every 2-3 days, this will ensure they continuously get the benefits from these probiotics before they leave their body.