Gentle-to-rolling trails for pulk...
I would think that you're on the right track with Tripoli or Livermore Road -- steady grades with no sharp climbs or drops. Depending on your stamina, you might try Livermore to the Big Pines trail (though a good section of Big Pines would be a carry, now that I think of it). Livermore to the Boulder is a nice short hike. Livermore the Boulder with a spur off on Greeley Ponds (to one of the bridge crossings, for example) would be even better. Tripoli up into the north end of the WV trail network would allow you to stop off for hot chocolate at Burt's Yurt.
I haven't snowshoed it, but if you're ambitious, the Cascades Trail (leaves from Snow's Mountain parking area) is generally rolling (though there are some pretty good whoop-de-doos), and the cascades are very pretty (even the very first you come to -- climbing up from there with little ones in the winter would probably be nutty). I think it's about a 3-mile r/t to the first cascade.
Don't completely rule out some of the WV groomed network, either -- Saturday we took our 10-year-old for an hour loop, out along the Criterion trail, etc... less traveled than the easier stuff on the valley floor, pretty, and it would be very easy to pull the pulk on groomed corduroy (and less jouncing for the little one, too).