Hi there,
My daughter is also 7 and she started swimming with the school last week. She also does a 30 minute lesson on a Saturday morning also at the local baths and I go with her. Her levels are typically fine and I usually just give her some pringles or a biscuit afterwards.
For school, as she does her lessons at 9.30am .... I adjust her breakfast insulin. She's usually on 1:12.5 but if her blood is less than 6 on waking (which on a normal day is ideal) I change the ratio slightly (to 1:15) so she's not running too low for her lesson. Then she has a couple of biscuits and a drink when the lesson is over .. sometimes it pushes her levels a little higher at lunchtime but she had a nasty hypo once when attending a swimming party which scared me - she was 9 before she went in and half way through she looked at me and said she felt funny (I was sat right at the side of the pool watching her every move!) and her blood had dropped to 1.8 which is her lowest hypo yet. So, this approach of keeping her blood slightly higher before her lesson makes me feel more comfortable for when I am not with her.
Hope this helps. I am sure she will have a great time. Let us know.