We had one bought for us for a younger child like yours. The suit stretched and the floats went above her head. It was the smallest they made. I researched the suits and across the board they are not safe. Putting her in something that is a float with a seat in it with an adult beside her is the best way. It is flatter and her head won't tip it over. Younger kids heads are the heaviest part of their bodies, that is why when a child tips over into a bucket or toilet bowl they cannot get out. Their head is way too heavy for their necks.
So, in my opinion, and what I did, was get some of the items I am posting below. The kids could not tip them over and we had babies as young as 6 months in them with an adult beside them. I feel they are way safer in the long run and those suits are just too dangerous.
This one is one I came across and if the kids were this small I would have to get it...
http://www.cozydays.com/pool-beach/infant-pool-floats/bug...
This one is totally cute too...
http://www.toysrus.com/product/index.jsp?productId=4043759
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The one I bought for the kids until they were at least 4 years old was very similar to this one, I am posting a couple of different sizes so you can see how stable they are on the water. They are very wide and the child is down lower. They do not have the ability to flip it over.
http://www.intheswim.com/Pool-Accessories/Toddlers-Toys-a...
http://www.toysrus.com/product/index.jsp?productId=12623657
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Here's an article about the floatation suits that reiterates the dangers of them. My kids arms were forced inside the suits as the floats went up. They did not have the ability to pull themselves up and out of the water. The suits are not as safe as they seem.
A coast guard approved life vest with the straps fully used should make your child float on her back and not roll to her side. That makes me think it was not the right size for her or something.
http://www.cafemom.com/journals/read.php?post_id=1492258&...