I wanna correct the other anon. Gillian didn’t sign for only 6 eps because that was all the time the boys had off from school. That’s not correct. In England, and London specially, school continues all year. The boys would only have had August off. And like you pointed out, they have a dad who has to spend time with them. So Gillian choosing 6 eps probably had more to do with her schedule than the boys’ school, since they were in school during 80% of the revival filming.

She recently said during an interview that she tries not to be away from them for more than 3 weeks so I assume it could be a deal she has with Mark or some sort of custody contract she has to respect no matter what and it could be the same for him too. 

So of course she tries to spend as much time as possible with them, but their parents are separated, and with what she said recently too, I’m assuming they’re not best friends, so they probably have to deal with contracts and custody law, and I think sometimes, she has to be away from them when she could actually be with them, and sometimes, she can’t when she should. That’s life.