I second the mention of using Floragen. My daughter is 9 years old, and I started her on it about five months ago. She has lots of constipation/stomach issues, and it's helped take away the pains she used to have, plus it has been helping keep her regular. She also hasn't been getting sick, so that's a plus! Since she's old enough to swallow, she takes the pill. But as someone mentioned, the capsule can be opened and put in milk or mixed in food. For awhile, when my daughter was very sick with ear infections, I did give her L. Acidophilus; I just opened the capsule and dumped it into whatever she was eating.
With our Western diet and the toxins that get into our body, I think it benefits everyone to take some sort of probiotic. Yogurt and kefir are very, very good to eat, but actually taking a capsule of L Acidophilus will get a lot more into your system. I myself take an Acidophilus supplement from drnatura.com that has over 8 million strains of 9 different cultures, and I absolutely love it! It's called Flora Protect, and it's the best I've ever found. However, it's too strong for children. However, drnatura.com recently came out with a kiddie line, and their kiddie Flora Protect comes in chewable tablets that taste like cherry (like candy, according to one of their customer support people).