Wanting to buy a sleeping bag for my 7 month old dd. She is very small for her age (currently still in 3-6 clothing) but I feel like a 3-6 sleeping bag wouldn't last long and I'd have to buy another one before the end of winter. The next size up is 6-18 months however which looks huge for her and seems like a massive range - big size difference between a 6 month old and an 18 month old I would have thought? I wondered if it'd be dangerous to put her in the bigger size in case she slips down into it? Is this possible if they have the shoulder straps?