Oh, Stiff, I'm so sorry about your estrangement from your son and DIL. I'm not clear, though, on whether or not you're saying that your GC aren't enthusiastic enough about your visits or that you're upset that son and DIL aren't warmer to you. If it's the former, please realize that little kids don't always greet people w/ enthusiasm. If all of you have fun on your visits, you're doing ok.
If it's the latter, if you're upset by lack of enthusiasm on the part of son and DIL, then I think you need to let that go. It's just not going to happen. You are, after all, estranged. Sad to say, that hasn't changed just b/c they let you see your GC.
As some posters have indicated - and as you'll see if you read around the threads on these topics - often when parents are estranged from their AC (adult children), the parents/GPs don't get to see their GC either. In those cases, the GPs often put aside money for the GC, etc, as you're talking about.
But you're fortunate enough to get to see your GC. And to give them gifts in the here and now, etc. As notanan says, if the pickup and dropoff are too stressful for them, then, by all means, bring the visits to an end. For that matter, if they're too stressful for you and DH, you might still want to put a stop to them. But if the pleasure of the visits outweigh those few moments of stress, then I hope you decide to continue those visits. Please just focus on enjoying your GC and, as hard as it may be, let go of any expectations regarding son and DIL (I know you'll always love your son, but that's not the same as having expectations of him).