They did an excellent job alone, Franklin is a minor and his parent were pretty clear about their position, Kitty taking Franklin against their parent wishes just because the BOY is curious is actually kidnapping, as a responsible adult her duty was sent the boy back to his parents.
The FF have no obligation to be "diplomatic" wh a group of fanatic supremacist came into their home trying to recruit their UNDERAGE son to their cult and menace his family. Yes some of them had worked with the fantastic four who makes their betrayal even more painful and infuriating, but they are in no light "allies", they came into their home as a racist group trying to take her son away of his family because his genetics. And that supremacist discourse made for Magneto not only let clear what their position was, but is utterly ridiculous. F4 are also powerful even when still human and also create the multiverse that all the living creatures inhabit nowadays, basically is calling lesser being at those who saved his live.
If they wanted a nice conversation they could have sent Kitty or Storm alone and talk, but the moment they try to make a force demonstration by sending somany people and many of them dangerous and aggressive and plainly racist against humans, they just show that they are not presenting themselves as "allies" but as a menace
Actually If I were Susan and some crazy supremacist comes to my home trying to brainwash my boy into promises they can't full fill, just to get him away, using him as a weapon and probably let him die once they are done with him (even if they can resurrect him letter), insults my family because their genetics are different, and makes menaces about taking my son without my authorization. I would be already making my husband work in some weapon able to make the M-pox look like spring rain, If something their response was very tame.