If the whole family comes there will be a press field day. Lots of coverage, probably all positive at first, look how much Lilbet looked like Diana, isn't Harry wonderful with the vets etc etc. If they meet Charles expect lots of doting grandpa articles.
But in reality I don't think anything will have changed.
Harry always does these Invictus trips, the fact he is rolling out the children isn't an indication that things are going well for them imo.
It will be lovely for Charles to meet his family who have "made their lives abroad". Who wouldn't want a family reconciliation to some degree, family splits are awful.
They'll get security because, well, there should be security appropriate to the threat level.
And very soon the press will get tired of writing positive articles, spin the wheel and use the same photos and stories in a negative context. Lots of lovely material they can spin out for months.
But none of that is any reason to think that Charles will want Harry back as a working royal. He isn't needed and it would be a headache for everyone trying to manage him. They will come over, get press, merch clothes, show off the children and them go back. If they are wise they won't give interviews about any meeting with Charles and any non meeting with anyone else. But will they be wise?