With strong available light, or good natural light, you can use the Mamiya with Delta3200 (or 800 color print film), and still get very enlargeable pictures. This setup allows you to move freely, and to always have an eye on the target, rather than the blackout effect of the SLR, and the feeling of tracking them through a peephole. However, this is from a person who is an obligate eyeglass wearer and likes the large GG, so take it with a grain of salt.

You can improve your success rate by slowing down your baby. An old trick which I've used, and passed on to friends who agree that it works well, is to put a piece of invisible Scotch tape on their finger. The camera won't see it, but they'll be fascinated for quite a while. Zone focus, and watch for a good expression.