It turns out that about twenty minutes into the process I decided to let them finish the puzzle first before continuing since I've interrupted them every other time we've done this and I've worried that this has detracted from the experience. This turned into the entire session for the day. In my defense, I usually finish the puzzles in about 15 minutes and I thought that 2 minds would be better than one. However, I never had a chance to time my own son doing these at home and so I'm glad to see their relative difficulty more clearly.
8x8 Kenkens are too difficult for a warm up and general strategies need to be discussed more/brain stormed. I went around a lot giving hints on ideas for forward progress but I'm not sure how many breakthroughs were occurring. I'm going to move onto different warm up exercises for the rest of the year. If I bring these back next year they will be at 4x4 and 6x6 level.
I did end up seeing several new and interesting partnerships during the hour. So shaking things up partly worked.
About a 1/4 of the kids were not engaged and needed a lot of prodding to keep on target. Maybe next time I'll let them self-choose new partners so there's less tension if they get someone they don't want to work with.
Anyway, I have all the rest of the planning which I will bring out again next week. I'm also thinking I will put out a call for other parents to help out. More adults/kids looks helpful.