Oh dear. So much love going out you only reasonably expect the same amount coming in. I don't know for sure but I'm in the position of the Other GP - kids spend a whole week with me every five weeks because of DS's work pattern, but see their other (much richer spoilier) GPs once a week or fortnight. They do share a lot of time on Skype, and DGs talk about them and what they did together to me, but when they're poorly or parents aren't available, it's me they want.
Sorry if that sounds smug. It's not meant to, but it's coarse proximity, what they're used to. They're only little and prob don't understand quality/quantity. I'm sure that when yours get older they will cherish the relationship they have with you and appreciate it with maturity. It does seem unfair!