Worsted weight yarn is yarn that knits at a gauge of 4 or 5 stitches per inch. The ball band should say what the yarn weight is. This is a very common weight of yarn throughout North America. Big Store brands like Red Heart, Patons, and Lion offer lots of colors in worsted weight yarn. A smaller local yarn store would also carry a variety of worsted weight yarns, as they are so commonly used.
I used to find crocheting easier, as that is what my Mom taught me to do. When I was first learning to knit, I had to force myself not to slip back into crocheting as it was easier when I was first learning to knit. Allyson, your girls are lucky to get to learn at a young age! perhaps they won't feel as dependent on one type of fiber craft.