We had the exact same issue with memorization when my son was learning to read music. He'd play something and memorize it almost immediately. We found the following sight reading books.
http://www.mnmusicteachers.com/productdetail.php?id=102 things with this book. These are all one line pieces, so it's easy to say we're going to sight read one a day and that's it (my kid was not a fan of learning to read when playing by ear was so easy at the time). They are usually quite random notes. So they aren't in a nice tune like pattern that is easy to figure out. When I sensed he had memorized one, we'd just move onto the next one. By the time we had worked through the 4th book, he had moved on to reading all his music. Now to the point that we are working on a piece, we have to work on memorizing it.
Morning practices work well for us too. I'm not sure either of my kids could take a music practice after a morning of swimming. Sometimes things that take twice as long when they're tired!
My daughter (who is close in age to yours) starting having a horrible time practicing this summer! It was taking an hour or more to get 20-30 minutes worth of practice done. And much of that was perfectionist dramatics ("So hard", "I can't do it"). I had to lay out a incentive plan for her that's going to end in a $10 toy if she can get 100 beads on a string. I put a bead on if she does something without complaining or throwing herself on the floor. I get to remove one if she is unwilling to try something when asked. We had one excellent practice this morning!
Anyway, I know exactly where you're coming from, so if nothing else I'm sending sympathy. Summers can be hard!
