I would be irritated that they did not inform me. Perhaps your son didn't seem to be bothered by the stuff in his eye, and after rinsing he was fine. At that point, it would seem moot. And really.. at that point (when it happened and they were rinsing his eye)... not sure I would WANT them to stop caring for my child to call me in that moment. And once that moment was over, if he was fine and not complaining, then there isn't a need for a phone call, really..
I would, however, have expected them to mention it when I picked my kids up. An, oh, by the way... x,y,z happened, but he seemed fine after we a,b,c... just so you know.
For the record, this exact thing happened to my son (at our home, not the grandparents). He was in 4th grade, so what's that... 9 years old?
He had a glow stick under the sheets in his bed after getting some at a party or something. He broke it and the stuff went everywhere, and yes, some in his eye. We, too, rinsed well with water.
I happen to have family members who are nurses, so I called and said, "hey, do I need to take him to the emergency room or anything... we're talking about an eye here.." and was told IF blah blah... then no, no ER needed. But if it was still irritated to have him seen the next day.
Why? Not because of the liquid inside... but b/c the way they work is a chemical reaction between 2 substances, and they don't glow until they mix and react together. How do they keep them separate before you snap the tubes and make them glow?? With little GLASS capsules. That's what that popping sound is when you crack the tubes before you shake (to mix the liquid) and get glowing sticks, or bracelets or whatever.
The liquid can stain your clothes, but rinsing from the eye immediately it is harmless. But if a broken fleck of that glass gets into the eye, it can scratch it.
I called my son's doc the next morning, just to be on the safe side. He said his eye still felt a little dry (like if he got soap in it, the next day after having done that).. but it looked fine, but just to be safe I took him in.
The doctor administered some sort of glow in the dark drops to his eye, turned out the light, and used a special device to check his eye for scratches or any pieces of glass that might have remained (he'd have probably felt it if there had been, but there wasn't). His eye was perfectly fine.
So for any of you who hadn't thought about it... if they break.. dispose immediately. And don't allow the kids to play roughly with those things. Harmless when they are sealed, but tiny bits of glass capsule pieces are floating inside with the glowing liquid.
And... THAT is why I would be a little irritated with the inlaws. Because if he complained about his eye later, you wouldn't have known what had occurred or how it had been treated, or known to WATCH for further irritation. Maybe they felt your son was old enough to tell you all that. But in my opinion, if my child is not old enough to be left unsupervised, then the person(s) doing the supervising are the parties responsible to tell me if something happened. Even if there was no harm done.
Something in the eye is not the same as getting dirty in the yard. Not by a long shot.