I think you have to speak up. Clearly, the person who suggested this idea didn't think it through at all considering that you don't have children.
It is very fair for you to say, "In retrospect, I don't think the idea of only buying for the children is going to work out since we don't have children. We would be buying for X (number) of children but not receiving anything in return. However, I understand that maybe buying for everyone is getting too expensive." Then you could suggest a Dirty Santa or a Secret Santa amongst the adults and whomever also wishes to buy for the children could do so on their own.
Oooohhh... I really don't like this. It makes i seem like Christmas giving is a tit-for tat thing, when the issue is really that OP feels left out of the gift exchange (or so it seems to me).
I would say something more along the lines of "I was hesitant in speaking up, but this has been weighing on my mind. I've been thinking, I really enjoy the Christmas tradition that we have established and I would miss it terribly if we were to do something different this year. In fact, I had already purchased a gift for (fill-in-name) and something else in mind for (XXX) and was SO looking forward to seeing your expressions when we exchange gifts. Also, not having any children ourselves, DH and I would be missing out on the joy that you will be able to experience watching your own children open gifts. If it's okay with everyone else, we'd prefer to leave our traditions intact."
One of the reasons why your solution struck a nerve for me is that I have a friend with a son the same age as my kids. Every year she made a big deal out of us getting together so our kids can exchange gifts (when I would have just skipped it. My christmas list is family only, and then any firends I FIND something for, I will buy it, but no one is mandatory). This was fine, my kids gave hers a gift and her son gave one to each of my boys. One year she says her son complained b/c he has to buy TWO gifts (one for each of my sons), and that's NOT FAIR b/c he only gets one in return (given jointly from my kids). So she asked me to make sure that my kids get his separate gifts that year.
I was shocked and appalled b/c I don't see gift exchanges as a reciprocal thing at all (as in... just b/c I bought you a gift, I do not expect you to buy me one, or to make sure it is similar in value, etc... and would rather get no gift at all than one bought as an obligation). I thought her son's complaint was understandable at his age (I think he was 7) and that it would have been the perfect time for her to teach him about christmas being about giving and not receiving, etc... but instead, she validated his complaint and wanted me to do so as well. I told her I thought it was best if we just didn't worry about the gift exchange that year. (and for the record, neither I nor my boys would have been upset had they received a single gift addressed to BOTH of them in lieu of a gift each). We have not exchanged gifts since, and this is the 3rd Christmas since then.