Based on the many recommendations in a nearby thread, I just took my 6-year-old goofball to O'Sullivan's in Somerville.
It delivered as advertised. The burgers were well-made and large enough to give rise to their own gravitational fields, and, being nearly spherical, looked like delicious, meaty planets. It's an exceedingly pleasant and cheerful neighborhood pub, and they made father and son feel welcome. They don't have a "kid's menu" per se, but the prices are low enough that you don't care. Lunch for 2 with one beer (Harpoon IPA on tap, hell yeah), one milk, and enough burgers, fries, and onion rings to ballast a destroyer came to $25 + big tip.
The onion rings were excellent, by the way, with a mildly sweet tempura-like batter, and fat, thick, legit slices of onion within. The "fries" are really huge sections of russets, expertly fried. Not crispy, but not greasy, with cubic meters of fluffy potato within.
For the sake of completeness, I'll admit that I'd prefer the hamburgers at Orleans in a blindfold taste test--they're so ^@!&!@#ing good--but O'Sullivan's is, for me, just a nicer place to visit.