Dr. Parker at WebMD (who I heart) explains why it's so important to put enough sunscreen on our kids, and to reapply it throughout the day. He even used the word "obsessive," which, coming from him, says a lot. I've been known to "forget" the sunscreen when we'll only be going out for a short time because it's such a pain to get on my squirmers. But I've been duly reminded — there are no exceptions.
Dr. Parker's recommendations go beyond the usual "reapply after swimming and excessive sweating" — be sure to read the article. And note: sunscreen marketed to kids differs mainly in price, not formulation.
In the spirit, I found this nifty UV-protective stroller shade on Amazon — I've never tried it, but looks useful. Back when I used a stroller every day, the sun was always at the perfect angle to blast my baby no matter how much I futzed with the built-in shade, so this looked like a good alternative. (If anyone has one, let us know if it's worth a buy.)