AE_bb,
I have not joined a tour with children before. I don't know how it feels (morning call / children want to go to toilet at an odd moment etc). If u / your husband drives, it is an excellent idea to go to Hokkaido by yourselves. Mid July is the best time to see lavender. You won't regret.
Last night, my husband told me that he really enjoyed the last few trips with our elder son - I did all the planning & he needed not worry a bit. Most importantly, I always allowed time for him to do shopping!