I would compare cost for Phoenix and Las Vegas, both for the flight and for the rental car. In the past, Vegas was much less expensive than Phoenix. The drive from Vegas is a little longer, but you can stop at Hoover Dam going or coming.

Also look at cost for flights to SFO from both cities when making the decision.

The baby is small enough to carry in a back or front carrier if you want to do a little hiking in GC. I would try to plan flights at nap/sleep time if possible.

Be careful driving around the Grand Canyon in February. Because of the high altitude, snow and ice (on the roads) is not unknown, and it will be quite cold at the rim in any event.

Exactly what emalloy said. Phoenix is a bit closer but usually more expensive for flights and cars.

Sedona and Flagstaff would be the closest airports but the time you'd save by not having to drive very far would disappear by having to make a connection to get to either airport. At they would almost certainly be significantly more expensive

From past experience I know Flag and Sedona would be much more expensive then either LAS or Phoenix. From Seattle it is likely cheaper via LAS and you can add Lake Mead, Hoover Dam and Vegas to your trip.
Snow and ice can be a problem in the park and on the roads leading there. I-40 is almost always open, but you can get storms that time of year that will be travel interesting.

I'll be travelling with my husband and 6 mos. old son.

Perhaps February isn't the best time of year to make the Grand Canyon excursion.

HTtY