I think most rational people would understand that having two children under the age of four travel twelve plus hours on a transatlantic flight without either parent would, in and of itself, be traumatic for the children (even if a trusted nanny or M's mother travelled with them). And why drag two very young children to the funeral of a great grandmother they never had a close relationship with? With all the other family drama and stress swirling around, hopefully they leave the kiddos home to enjoy their normal routines. JMO.
I can see the pros and cons of bringing the kids over for the next week or however long Harry & Meghan remain in the UK post-funeral. On the one hand, yes, putting the kids on a transatlantic flight without a parent would be a huge challenge - if they were flying commercial. But, they're not, so it's a much more manageable proposition, even if it is still not something anyone would relish or look forward to. On the other hand, they were only planning on being away from home one week and both they and the kids must miss each other. I can see them wanting to bring the kids over, especially if there has been even a slight thawing of relations. There will be private gatherings, I'm sure, and opportunities for Archie and Lilibet to interact with their cousins (not just their first cousins, but the rest of them as well).
I'm curious to see what children do attend the funeral. I think George definitely will, and quite possibly Charlotte. I'm not sure how old Zara's children are but if they're mature enough will probably also attend.
My guess is we will see the following: Savannah Phillips (age 11), Isla Phillips (age 10), George (age 9), Mia Tindall (age 8), and Charlotte (age 7). IIRC, those are the five who attended the remembrance service for Philip last spring.
The other great-grandchildren (and step-great grandchild) are all considerably younger: Wolfie Mapelli Mozzi (age 5 or 6), Louis (age 4), Lena Tindall (age 4), Archie (age 3), August Brooksbank (age 1), Lucas Tindall (age 1), Lilibet (age 1), and Sienna Mapelli Mozzi (turns 1 on Sunday).