Well, those testers are not all that accurate between 2 and 5, so maybe you started off with a little less than 5, and ended with a little more than 2? If your CYA is still low, then that could also account for the big swing. Hot out? Maybe a couple kids didn't get out to use the bathroom? There's some more FC. Doesn't really matter, now you're back at 2.
The kids splashing can raise pH. I'm not sure how much that bubbler gizmo is going to help. Some, yes. I believe it's like a bubbler in a fish tank. It's not the bubbles themselves so much that are oxygenating the water, but the bubbles breaking and rippling the surface that increases the oxygen exchange at the surface. So, yes, your bubbler is helping a bit if the bubbles are rippling the surface. You can see in my pic how much water action I used to ruffle up the surface. And I remember having to run my pump a lot to get my pH to move. Can you bring that LED gizmo online? Does that ruffle the surface? Do you happen to have any sort of pump, like a sump pump, that you can put in the pool? Set its output hose to come out of the pool and splash water back into it?
Yes, I think the pool cover will inhibit the pH effect, but then it's also helping to hold your FC from losing to UV? Not super sure about all that, or which is more important at this point. You're challenging your pool by opening it, and allowing swimmers, without the inventory of chemicals you need to keep the water balanced. What can I say?