Open Forum

Hello everyone! This is my first year teaching theater to a group of homeschool children at a weekly drop off program. I see the children all on different days, and only have about 45 minutes with each group per week. I have been writing a show for them to do, but am realizing that if we do this again next year, I might need to do a small play for each grade group, which is something my predecessors all did. They also put on a big show, but overstepping their rehearsal permissions. I am trying to set a positive precedent with our space, but am feeling as if  have bitten off more than i can chew. 
I have been problem solving for the last 8 weeks, trying to adjust to the parameters. I have grades 3 and under all doing a very short play we can rehearse fully in class, as I realized most of them could not endure waiting backstage for long. Very wiggly!
Grades 4 and up are doing the 2 hour show. Our venue won't let us do a lot of extra rehearsals (which is understandable.) What would you do if you only had actors one day a week for 40 minutes? I can certainly have them memorize at home, and practice with friends, but am going to ask very politely for one rehearsal day per week to have us all together running scenes.
HOW do you all break up rehearsals productively when you don't have much time with your cast? I don't want the children stressed about having too little rehearsal time but also want to keep my commitment to giving them a "big show." 
I would very much welcome input!