Unasked-for advice is tricky. No one can see what you see and know the parents like you do.
If you really believe the twins are delayed developmentally, I personally feel it's not only ok but ethically responsible to approach the parents, but not with just your dis-ease about the situation.
I would suggest that you start by doing some solid research. Find out everything you can (it's pretty easy) about developmental milestones, such as speech and walking. If you're going to mention their diets, research that as well. It's completely possible that the parents just don't know much about how babies are supposed to progress, and aren't concerned enough about it to compare their kids to others. It's also possible that you'll learn they aren't that far off the charts.
Armed with facts gleaned from multiple reliable sources (if you're still concerned after your research), ask the mom if you can discuss some concerns you have with her (or, alternately, both of them). Just tell the truth, but make it about yourself, not them.
This is just my fallible opinion, but I'd approach it something like this:
- I'm sorry to be so dramatic, but I'm feeling a bit uncomfortable about (baby x) and (baby y). In my limited experience, most babies (cruise, say their first words, dance the cha-cha) by now. I didn't want to overreact, so I looked at a few pediatric websites. I hope you don't mind. I did find (this) on (these websites). I would be so relieved if maybe you could run this by their pediatrician on their next visit? (They should have a 15 or an 18 month check coming up.) I wouldn't feel right about myself if I didn't bring it up.
What I would not say:
- I'm concerned that (baby x) and (baby y) aren't (cruising, saying their first words, dancing the cha-cha) yet. I've taken care of a lot of kids, and I think they're significantly behind on their milestones. I don't know (Dr. Doofus) but I can't understand why s/he hasn't said anything about it to you.
The first is putting a bug in their ear, showing empathy for the concern you might be causing them, and is in keeping with your respective roles. And you can occasionally feed the bug if at first you don't succeed.
The latter, while it's straightforward and true, is a lot to give people suddenly. When people are anxious (which they will be as soon as you get it out), they have a natural tendency not to hear what you're saying and sometimes to react emotionally, not reasonably.*
Whatever you decide, I hope it works out for the best. I think it's great that you care enough to bring it here.
*I've learned this the hard way early and repeatedly.