No not a Naff idea at all, my DD had exactly the same fo her 2nd B'day too, & it went down a treat, though it was a Summer party so we held it in a local park, length of checked gingham PVC table covering on the floor as a picnic mat...you'll find it on your local market haberdasher, or any curtain supply place, & games such as "pin the nose on the teddy bear"
Don't go too over board, you'll find its not really that necessary at that age, just a few simple nods to the theme & invite the Kids to bring along their favourite bear.....
we had a Teddy Bear Cake, & I made up a "picnic" of individual lunch boxes, Though DDs requested cooked broccolli, Carrots & fish nuggets were moaned about by a few parents 
so probably best at that age if they contain a few favourite finger foods, such as carrot & cucumber sticks, cherry tomatoes etc, a small finger roll (or some sort wheat free wrap if an allergies to cater for) & maybe a tub of berries such as strawberries, & a carton of smoohy or juice
don't go mad, they'll be too keen to play to want to eat much, but I would recommend sticking with natural stuff as a lot of kids can get quite hyper at that age with too much sugar or additives, so it will help avoid too many squabbles & tantrums etc
If you can't find a cheap small bear, what about getting a teddy bear biscuit cutter, & making teddy bear cookies, you could maybe even mix up some icing & have bowls of spinkles & sweets etc to decorate, & let the kids decorate their own as a party activity & put the finished 1 in the party bag, or if that sounds too much, just make & decorate your own & add the the bag.
If the kids do get to manic, or even just to finish off the party a game of "Sleeping Bear" ....which is just "dead fish" changed to the party theme....ie play music & the kids who keep quietest & stillest longest win, & those who make a noise or move are out
good luck & enjoy :)