Firstly, important info missing here is how long they’ve been taking lessons and their ages.
I’ve supervised swim lessons for kids camps for years. Kids ages range from 2-15. They all varied in experience and maturation. Yes, there are some typical coordination differences as mentioned, and body fat differences.
From the sound of your question the kids are very beginners. I’ve seen 2 year old girls whose siblings were both genders who wouldn’t even get in the water! Screamed and refused. So from that perspective, I wouldn’t worry about it and instead, value their uniqueness and let them enjoy the water together.
(Aqua Fitness Pro)