Do we know that M won't allow the children to visit the UK or is it guesswork?
I reckon she won't allow it unless there's something in it for her, in a "I can't believe I'm not getting paid for this" way. She has something that she believes to be very valuable both financially and in terms of her standing: her children, a prince and princess. In her mind, the opportunities are limitless. She's been taught by her mother not to "give the milk away for free" and my goodness doesn't she live that way.
I mean, Harry had a minutes-long meeting with his dad after a year and a half of estrangement (a meeting which, by some accounts, didn't exactly go in Harry's favour) and within 36 hours Meghan was on Instagram plugging her rose with a direct reference to that meeting (she'd argue it was his general absence of 4 days but let's be honest, if KC3 hadn't figured with all the Sussex-generated will-they-won't-they media coverage beforehand, she wouldn't have referenced the trip. She hasn't before, has she?). If that's not monetizing the royal connection, I don't know what is.
I've found over the years that the pro-monarchists on this board really struggle to wrap their heads around the enormously different world Meghan comes from and now lives in. It's a massively different culture and way of being. People underestimate the extent to which money is the beginning, middle and end of everything. It's the one thing - unlike class or reputation, for example, in the UK - that will always be the final arbiter of who a person is in the US. It took me years and years of living in the US to really grasp the extent to which this is the case.
I can only think Harry is still struggling to understand it himself, and not only because he's not exactly the sharpest tool in the box to begin with! His very lukewarm reference to "I like my life" in the Guardian interview was very telling. Meghan is for Meghan first, last and only. It's anathema to a man who says that he's lived his own life in service to the British public. I think he knows he needs money; but I have a feeling he struggles on a daily basis to accept what's required to get enough of it to maintain the life that he wants, let alone the one that his wife wants.