Kids Sleeping Bags… do I really need to buy one?

You might try to use the existing sleeping bag (if it fits well) and just get more clothes. If you were starting from scratch then this wouldn't be such a great idea, since it is much more efficient to add filling to a bag. But your other idea probably wastes even more (I think). The advantage of carrying extra clothes is that it gives the kid more to wear if she goes out at night or in the morning. I would make sure to get a really good, fuzzy hat (the best warmth to weight item), some light weight glove liners and an extra pair of fleece socks a couple sizes bigger than her current size. The fleece socks should fit loosely over her current socks. They might only be used in the sleeping bag, but will help if she gets cold feet. Extra long johns and sweater aren't too expensive either and she could use them in the winter.

Thanks Ross
I will try that again in Feb when we go up to test out our gear again. It has been getting down to 28 degrees. So if my granddaughter can stay warm up there in the just extra clothes, I will just bring her bag.
Great idea about the extra fleece socks!

The baby we figured out is wearing her snow suit and I am using a down jacket for her sleeping bag!! :D

Are you backpacking or car camping with them? If car camping I'd just use an adult bag for the older one. The down jacket works great for the baby…I did that with my son.

If you are car camping then heck, don't worry also…just bring extra blankets to toss on top and under the kids if they get cold.

Wonderful…I thought the down jacket would work out, happy to hear some one else has done that too.

We are backpacking on the JMT unless there is just too much snow, then it will be car camping and playing in the snow!

If you have a good adult sleeping bag, they are often lighter than kids ones anyways :)