I don’t believe they want to take the risk of bringing their “little ones” back to UK for fear of DNA results being revealed on the tykes.
I believe it is true (Brits please correct me) that HMQ has certain rights over the persons of those who could succeed her.
This includes (at least for now) Archie and Lilibet, and it’s the reason Prince Markle Sparkle and the Mrs. cannot run the risk of transporting them to the UK.