Babies have such different developmental curves, we spend a lot of time stressing ourselves out for nothing. First, I'd say that's it's sometimes hard to remember that the milestones are often later than we remember them to be. For a while, I was concerned that my almost 2-year old wasn't seeming to "get" colors, even though she could name shapes and even some letters and numbers. I worried because I wondered if there could be some colorblindness issues. But when I looked into it, the literature explained that colors were more conceptually difficult and were only a concern after THREE! Now at 2 yrs and 2 months she's totally got it and I was worried for nothing.
The time frames that actually constitute developmental delay, as opposed to normal variation, are generally WAY later than we think they would be, and pediatricians are on the watch for this stuff, especially for preemies.
I think something else worth pointing out is that grandparents need to stay sensitive to the fact that parents are already stressed and on top of this stuff, and that as tempting as it is to get into it, your job is simply to be encouraging and supportive. My first daughter was a slow weight gainer and infrequent pooper, and when older late to potty train. One set of grandparents were calling us constantly wanting to know what went on in pediatrician appts., what we were going to "do" about it, and constantly telling us she was not normal (which wasn't even right). They went so far as to tell my husband I was endangering our baby by breastfeeding her. Needless to say, they were wrong on all counts, but we were already stressed out enough over this stuff, had a great pediatrician and our relationship with those grandparents still suffers to this day. So I would gently encourage you to work out those worries with a friend, and not with your son and daughter -- with them, just be an ear to talk to about THEIR concerns... and if they aren't concerned, let it be. As long as they have appropriate professional guidance, trust it and trust them.
Please accept this in the spirit it was intended...as a parent who has been there, not as a "scolding." I think it's great when grandparents are really interested in their grandkids' lives and well-being, but it's hard (I imagine) to switch parenting mode off, and grandparenting mode on. :-)
Warmly,
Nicki :-)