I was going to suggest exactly what Tan did!!!!

I have used the grobag for both girsl. Marley is nearly 16 mths and still uses the 6-18mth bag, with room still at the bottom. I have a 2.5 tog for winter and a 1.0 tog for summer (got a great deal if I bought 2). Given we are in spring, I would get a 1.0 tog and if need be, add extra clothes on overnight if you think your DS is going to be cold and it will be the perfect weight for summer. I've never heard of the dream bag, sorry!

Good luck.