The difficult to understand part is why it would be better to move to where he lives as it’s going to be much more disruptive for your seven year old to move house and area and schools and have a mother who is grappling with a new job than it is for his twelve year old. Actually I think it’s harder for the twelve year old.
Yes the OP’s DD would need to change schools, but children do change schools all the time and they really do adapt. Plus at this age friendships are very fluid anyway, changing friends at the age of seven isn’t likely going to scar the child for life.
But for the twelve year old, one of his parents deliberately moving away from him to set up a permanent home with someone else’s child is likely to lead to all kinds of feelings of rejection, e.g. if his dad can no longer see him on weekdays because he has to rush home to somewhere else.. He is going to be the one made to feel second best.
As for the poster accusing the OP of only wanting a shag, OP I would just ignore that. Yes, children absolutely do need to build resilience. Not just in this situation but in others. If the OP was moving with the child’s father and she was going o have to change schools she would have to do so. It’s ridiculous to suggest that a seven year old should never have to change anything in their life. No wonder so many kids grow up afraid of anything and everything and with anxiety if this is how they are mollycoddled when they’re little.
Yes of course there needs to be some thought process around moving in together, and OP has already said they’re not planning to do it for another year or so, so moving in with someone after over two years is perfectly normal.
I moved countries when I was nine, went to a boarding school where I didn’t speak the language and saw my parents first only at weekends and then only in the holidays.
I am now a fluent speaker of said language and most of the friends I made there are still friends of mine to this day. Children have differing experiences in life. I think that a lot of people are so anti this one not because the child is going to be affected but because there is a man involved.