Interesting. I knew about the stew, of course, but not that it worked on nursing mothers as well. I'd say this is a feature, rather than a glitch. Arthur may not have specifically meant for them to be able to gain skills in this way; similarly, he may not have intended kids to do the same. However, I'd imagine he didn't specifically mean them to be completely unable to gain skills while nursing. It's a by-product of allowing nursing mothers and kids to eat stews.