My first concern was the puppy in the awning. Fecking freezing!
Same here! I'm a veteran camper, and when we've had a puppy (I've had 3 over the years) they've always slept in with us. Even in summer, the nights can get very cold.
Puppies are wriggly little buggers and can escape a tent or awning with remarkable ease, even if it's got a sewn-in groundsheet (yes, the sneaky beasts can undo zips!).
I also wouldn't want a young dog to stay with someone else for 5 days unless it was someone I knew well enough to know that they treated and trained puppies like I do. Even then, I'd be reluctant to do it unless there was no alternative, like I was in hospital or something. The ILs may have very different standards when it comes to acceptable behaviour and may even be old school and believe in hitting puppies when they do the "wrong" thing.
And they are BU for arranging all this without telling you. A 3-year old is too little to be away from home and parents for that long imo and, while I loved camping trips with relatives when I was a kid, I didn't go away without my parents till I was at least 6.
How do the older kids feel about this? Are they looking forward to it?
Is there any way they can all go just for an overnight, and you could drive over and pick them up the day after the event you're going to?
I might be minded to find a diplomatic prior engagement that involves the children on a day that's in the early part of the trip, so you have good grounds for going to pick them up from the campsite. or, if that's not feasible, make other arrangements.