Is this a new thing, or have they always behaved like this when you visit?
You say they have received a good bit of help from you, could guilty conscience be causing them to behave like this to you?
I don't know where you live, but should you be visiting right now? Perhaps you aren't but are talking in general.
It sounds to me, as if you should visit less and give them time to miss you, and when you get out of lockdown try to find some activities where you won't be depending on your children for company.
You are feeling hurt by their lack of interest, which I do understand, but the behaviour you describe is fairly typical of their generation.
So it begins….. Streeting resigns
