I've been feeding my daughter store-bought baby food in glass jars, but I plan on starting to make her food myself. Is it safe to wash and re-use the jars I already have?
Glass that is not specifically manufactured for freezing has the unfortunate ability to crack, leave behind tiny, microscopic shards and fragments and also is prone to bursting. Baby food jars are not manufactured for freezing or for extreme heating. BALL, makers of home canning products, manufactures 4 ounce jars specifically for freezing as do other manufactures.
So, for as long as you aren't planning to freeze (cold temperature) or start canning food (hot temperature) for long-term storage, you should be fine.
We used the baby food jars in the beginning, and now we use them for various things like carrying drinks (water/milk/juice) for our little one, or carrying snacks/food in it when we are out and about. It works pretty well for us.