I like Kayak. It searches multiple sites all at once (uncheck the "use priceline" etc. or you'll get a bunch of other windows opening, which is really annoying IMO), and lets you specify if you are flexible up to 3 days before/after your chosen date so will search on those too and you can pick the cheapest days. It doesn't, though, and to my knowledge neither do any other search engines, search Southwest, which I've had good luck with and I'm pretty sure they fly out of Boston. A big plus is that on SW bags fly free, whereas you can otherwise pretty much count for sure on paying at least $25 extra to check each bag (so don't forget to add that in when you're comparing fare differences). For Southwest as far as I know you have to go direct to their website.
Happy travels! If you want info. on a great (IMO) alternative to a car seat for travel with a LO, let me know.