I don’t think gift giving should make up for relationships. I understand the kindly motivation, you don’t want them to miss out, and you feel that they may lack from familial relationships, but presents, a momentary experience won’t make up for that. As they are now adults, that changes things. Most adults don’t receive a lot of gifts, they won’t be missing out.
I’d focus on experiences, spending time with them, asking them about their days, maybe bonding trips like going to the cinema or dinner, all things that build a feeling of family closeness.
I don’t know and don’t need to know your family situation, but if the falling out isn’t because of physical or emotional abuse, I’d let your children know that they can pursue that relationship if they are curious, just that you would like to be kept separate from it.
I’m sure you are a great mum and your kids know that, don’t over think it and overspend.
(You can tell I’ve done a lot of therapy over the past couple of years ?)