Okay, here's the opposite opinion of everyone else. (no disrespect intended).
In my medical anthropology class, we had a 3 week lesson in plastics. Microwave safe plastic is "safe" because only a small amount of chemical will leach out into your food. But it will leach into your food! Small doses may not have been proven harmful (yet) but from what I've learned through the research,(when looking at the research look at who funds the project- plastic companies have a big interest in keeping their products on the market) I stay away from it.
I brought my son's food in pyrex dishes with the nifty blue lids to be microwaved in and then had his teacher put the food on a plate. When I worked in daycare, we would microwave on paper plates and never in the plastic containers. You can ask your child's daycare if this is possible. If not, you might consider cold lunches.
I know others may think I'm going overboard but why take chances when they are so young?