If your children are indeed very young, Belvedere may be a bit harder to navigate with strollers, but on the positive side, it's very close to the services in town of TIburon and you can easily walk there. Tiburon in general also isn't known as a "walkable" town, as it encompasses a large geographic area (unless you happen to find a home at the tip of the Peninsula (near Belvedere) where you can walk to the restaurants and shops right near the ferry. You didn't ask, but down-town Mill Valley is known as one of the most "walkable" areas and is very desirable for families for that reason.
On a side note: The weather in general in Tiburon and Belvedere (and Mill Valley) has MANY micro-climates, depending upon where your house is. It's important to understand this phenomenon that many folks moving to Marin from other parts of the country don't anticipate.
Last summer, all of Southern Marin (Mill Valley, Tiburon, Belvedere and Sausalito) were exceptionally cold and foggy for the most of the summer. Towns just north, (starting at Corte Madera, Larkspur, Greenbrae, San Anselmo , Ross and Kentfield.are MUCH warmer in the summer). The winter weather tends to be similar wherever you live. If you do enjoy warm summer weather, this should be taken into account when you are looking for the best place to end up in Marin County.
As it sounds as if you are a mom to young children, I'd highly recommend joining the Southern Marin Moms Club. For a small annual membership fee, you can ask MANY moms their opinions of all the different towns in Marin. the link for the mom's club is: http://www.southernmarinmoms.com/