I believe CM talked about at this age doing the reading and narration to be done in 15 minutes. But don’t quote me on it!
I do just math games with my DS (5.5) and we do spend probably 20 minutes on it. But it is not focused math but more playing. Can you do an assessment to get a better feel for the level they actually know rather than having them do stuff that’s really easy? It seems to me that if they understand the concept of 2+2=4, it should be very easy to explain the notation.
How fluent is their reading now? My DS is very fluent. Right now he reads to me one article or portion of an article each morning from God’s World News. The rest he does silently, either free reading, or a small bit of assigned reading from the Nature Reader. Once they are fluent, don’t feel like you need to sit there and listen to them the whole time.
Plus, I totally understand wanting to take turns with them so they can each have 1 on 1 time. I just think you need to make use of the down time to have one doing independent stuff while the other is with you and then switch. Then you can all have downtime at the same time later.