Here's a little explanation I found on www.parenting.ivilliage.com:
There are two very important reasons for not overusing sunscreen in young babies:
- Infants and children have what is called a high body surface to volume ratio. What this means is that proportionately babies have more skin for the size body as compared to an adult. Sunscreens are made of chemicals. On the older child and adult, the chemical exposure is relatively minimal because the body surface ratio is smaller than a baby. Therefore, babies get a higher "dose" of sunscreen than do those older. While this exposure would most likely not cause any problems, the likelihood of an adverse reaction is greater.
- Babies have a mildly impaired mechanism to keep cool by sweating. In addition, they have a greater risk of becoming dehydrated due to heat. Sunscreen can somewhat impair effective cooling by perspiration, so placing too much of it on an infant could be detrimental.