As long as there is a place for air exchange, they should be fine. We all worry about our little finned babies, esp. that first winter (I have box turtles that dig in around now, and won't emerge until spring -- every year I hold my breath until I see that each and every one made it through the winter!). It's important to make sure the water is clean (bottom leaves & crud scooped out), before it gets really cold. Stop feeding at 50F. Make sure there is a hole in any ice that forms. But even when we do everything right, sometimes we lose some, and it's just not possible to be 100% all the time. So take a deep breath, and focus your attention on that cute little person in your picture! Love that little grin!