Whilst I absolutely sympathise with both Meghan and Harry surrounding concerns for their safety and that if their children. Realistically what they are asking is just to great.
Both adults made the decision to move to the USA. Whatever the reasons behind that move, which I don't know because I don't know them, it still happened, they moved.
The British taxpayer and the metropolitan police simply can not fund or deliver such high level security in the USA.
UK police officers would have no powers in the US. They would all have to purchase US guns and register them as per state law, they would then have to carry them as per state law. And I don't think California is a conceal and carry state, I think it's an "open carry state" and then, even after all that they have no power of arrest or detainment. No authority.
You'd also basically be asking for met officers to do some sort of overseas tour. Now many of them have families of their own, your asking them to go to the US on extended tours away from their families, when they have no jurisdiction, no power of arrest, nothing. Just walk around all day with or without a gun.
So if the couple want security the only option really is a private US close protection company. Now would I be adverse to the RF paying out of their own private, rather deep pockets for H and M to have security not at all. But again this would have to be a private close protection company registered in the US.