How should I approach this situation with my stepmother-in-law?

I have been in the same situation. I've received mis-matched Tupperware, clothing items with Goodwill tags and items that I know were previously owned. When I was young this bothered me. However, as I get older, the less the cost or "newness" matters. My thoughts about gifts is that they are just that: gifts. Someone thought enough to wrap something and give it to me. There are many reasons some people re-gift or give secondhand gifts and not all are meant to be spiteful. Addressing this will with your SMIL or FIL only make you ungrateful. I would just inspect your children's gifts carefully, explaining to the kids that grandma isn't used to small children and their safety needs. Just make the most of it and try to be a gracious receiver.