Good question and it would help me to have their ages!  Here are some general ideas though.  For babies and very young children, our office likes the Vive Sana 'Solar to Polar Baby.'  It's chemical free, water resistent and SPF 42.  I'm adding it to our top products in 2012.  Buy online.   One strategy is to use it on the key areas like the face and keep the rest of the body covered in light clothing. 

For children age 2ish and up, I really like rash guards in the water (like surfers wear) and light clothing to block on the body.  See Sun Precautions and Coolibar sites for ideas.  The really good sunscreens are too expensive to slather total body and the cheap ones have a lot of chemicals.  Hats are key.

Sometimes I'll have my children use the cheaper ones on the body and the better quality like the Elta MD sunscreens on the face, neck, chest and upper shoulders.  The Colorescience Powder is a product most kids like.  Don't forget the ears!  Get them in good habits early because when they reach the teen years they will only do half of it, if we're lucky!   Dr. I