I think, like a lot of people here, I'm struggling to see exactly what the issue is, as it's not clear whether you are the parent or one of the children, and your question isn't very well defined so it's hard to answer it.
I'm going to attempt to recap what it sounds like:
Parents have two successful children, neither of whom require financial support from the parents, and both of whom are financially comfortable and professionally accomplished, but DCA is a fair bit wealthier then DCB.
Parents provide more support to DCB in terms of childcare, which DCA resents (Does DCA even have children? You say they can afford childcare so I'm guessing yes, but the resentment isn't because they have an unmet need). You don't say what exactly they resent here, but if it isn't the money then it must be either envy of the time you spend with DCB and their children, or it's simple entitlement that they aren't getting something the other is getting.
DCA pays for parents to have expensive holidays and gifts such as cars, which DCB resents (because they aren't included in the holidays and expensive gifts? Or because they can't afford to match the gifts to the parents? Unclear). Again, it's not clear exactly what the cause of the resentment is, because you aren't providing anything to DCA here, so again, is it a time and attention thing? DCA gets to swank around the Maldives with you while DCB only gets to schlep around Waitrose?
Regardless of causes, the question I think you're asking is how do you not get stuck in the middle.
Clearly neither child is poor or unsuccessful. Neither child has a valid need that you aren't meeting. You are providing childcare support to one child that you aren't providing to the other, but you say the other child doesn't need you to. Therefore it seems reasonable that you are only giving this support to the one who actually needs this support.
You are also going on fancy holidays with only one child and not the other, but as that is paid for by the child themselves and not you, I don't see how you could be blamed for that.
Therefore, to me, it sounds like two fairly entitled adult children squabbling over petty jealousies that they aren't treated exactly the same despite having different circumstances and different needs. I hate this entitled idea that a lot of people seem to have that if you don't give your kids the exact same then you're not being equal in your love. My mum has always said that she loves us the same amount but that as we have different lives and different needs, we get different things based on those needs. What she does try to do is make sure there is some balance, so she might offer my brother more financial support and me more practical support. Or she might spend more on me, but more on my brother's kids, or more on me this year but more on him another year. She might go on holiday with him but spend the summer helping me have a clear out. It's not the same things but it is the same amount of love and consideration. It seems reasonable to me that while you have given child A your time and attention by going on holidays together, you've given child B your time and attention by offering childcare support.
However, to answer what I think your question is (how do the parents avoid getting stuck in the middle), I think you need to make sure that your children are complaining to the right person. It doesn't sound like you're truly favouring one child over the other, so this sounds more like sibling jealousy, so next time one of your children complains to you that their sibling gets more time/attention/effort from you, tell them you will always help your children if they have a solid need, and that this isn't always equal because need isn't always equal, but that they are both adults who made their own life choices and have more than enough resources of their own, so beyond that they need to grow up and if they have a problem with their sibling, to take it up with their sibling and not you. When people get caught in the middle of an argument it's important to realise that this is usually because the two sides are using you as a go between to argue with each other. Once you identify that their problem is with each other and not you, it enables you to stand back and tell them to take the issue up with the person they're actually angry with. If it helps give you some perspective, both my brother and I rely entirely on benefits to pay our bills - me because I have a degenerative illness and can no longer work as a university lecturer, my brother because he runs his own business while being a single father to 5 children and can't make ends meet. Neither of us rely on our mother for money and manage perfectly well on much, much less than a dual NHS consultant family and are both perfectly content with what we have. My brother does, however, live with my mother (but pays his way) and so gets much more practical support. This is because he needs it more than I do. I don't resent it because I don't need it, and because I love my brother so I'm grateful he is getting the support he needs. Either your kids aren't getting something they need (and that might be a good question to ask them - is your distress down to me not giving you something you need because I'm not aware of it?) and resent that the other is, or, and this is how it sounds, they're both entitled children who should grow up and stop envying what the other has and start appreciating what they themselves have.