I have used bifidus powder for my ds since he was tiny. I just mix it with water and give it to him on a spoon. If your lo is still breastfeeding, bifidus would be a good idea because it is specifically for baby (milk-digesting) guts.
Now that ds is older I also give him a chewable probiotic that our co-op carries -- Dinodophilus. It is sweetened with xylitol, which is supposed to be good for teeth and a healthy sweetener. It tastes very sweet, though, and ds didn't like it at first. He likes them just fine now. Since dd takes them every day, I just offered it to him every now and again.
When looking for a probiotic, whether it is chewable, powder, or liquid, you should look for one that contains several different types of bacteria, rather than only acidophilus, for example.
I also give my children plain, live yogurt. Ds only gets goat yogurt, though, because I suspect he doesn't do well with cow dairy.
You could add a powder or liquid supplement to a little yogurt or applesauce to feed it to your child.
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