Self isolation and support bubbles

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Asking because I cannot find a definitive answer to this. There’s lots of confusing advice!
My partner and I live apart (selling houses has been stalled by COVID!) We were planning to spend Christmas Day together at his house with our respective children. We both live alone so our support bubble is all well and good- have been together for over 4 years, all children get on etc etc.
This morning his son received a notification from school that he needs to self-isolate until Monday. He will be doing this at his DP’s house as he has arrived there for contact today. I haven’t seen the son since last week but have seen DP since he had his son at the weekend.

My question is, can we still go there for Christmas? Son isn’t leaving the house, since seen DP, he’s in our bubble... can we carry on as planned or do we need to stay home?
I just can’t work it out!
Thanks

I think so, the son can't leave the house but you can go there. If he gets symptoms though you will have to isolate too.

That’s pretty much what I thought. We do live as one household (albeit in our separate places) and have all self isolated in the past when one of us was symptomatic.
I know it’s perhaps not the best thing to logically with rising cases but the kids were devastated at the idea of Christmas alone. They are very tight together
Thank you