Think of their quiet calm life as a couple. Huge tracts of total silence. Perfect order. Everything still and tidy. Calm, unwritten routines going on. Contented understanding. They are older and get tired more quickly. They are less tolerant and get anxious much more quickly.
Then you and 3 kids arrive to stay. Even if they were practically statues, it’s an immense whirlwind for them. Noise, things being moved around, sticky fingers. They probably prepared for ages as well, moving delicate things out the way. Buying extra food, getting beds made up.
The walk was a nice idea. Your dad’s judgement was bad, but he’s not used to knowing what a 4 year old can cope with. If it had been a 7 year old that instruction would have probably been ok. If you’ve had a 4 year old recently then you’d know. He reasoned that it was a straight path so how could it go wrong. He didn’t judge the emotional intelligence of a 4 year old getting in a panic being on his own. He might well be an excellent grandad when they’re indoors but being out and about is very different. When was the last time your dad was outside managing a 4 year old in a park.
When they realised he was lost, they would have lost their minds. The thought of returning home without him to you would have been beyond the pale. Even thinking about a 4 year old being missing for 20 minutes, even 5 minutes, makes me feel physically sick. I can’t imagine how much that feeling would be magnified if was in my 60s and it was my own grandson. I’m quite surprised they weren’t physically vomiting in fear.
Looking at not talking to you/ apologising to you afterwards. Partly the event of course being enormous and totally out of their normal day to day life. But also partly that it feels unnatural to them to be apologising to their daughter and having to defend their actions. It’s a change of status and awkward. It’s their house and their choice to go on the walk and you’re their daughter but within this dynamic you are the head honcho because you have ultimate responsibility for those children and not them. But they don’t want to be answerable to you. Your dad probably does feel you’ve been ungrateful because he’s left with a dishevelled house, and his wife is in an absolute state. They possibly also feel a bit useless: in their heads they feel 30 and totally able to cope but they can’t anymore.
So ultimately if I were you I would honestly send them some flowers and thank them for their hospitality and reassure them that they mustn’t worry about DS1. Especially since it’s so out of character and they are normally such good grandparents.
From now on, don’t stay longer than 3 hours. They can’t really cope with more.
Your DS being told he was naughty for running off is not acceptable by today’s standards but back in their day, adults were not considered to be in the wrong and this, in their historical context, is absolutely normal I would say. Just look at all the paedo behaviour that was ignored or minimised at the time.