@Unreasonablyextravagant hats off to you and a bit of a slapped risk to the rescue.
I know it's an unpopular opinion - that it's hard to get a rescue because they normally stipulate older kids - it's for this reason. I have a very dog savvy 12 year old DD and even she is a pain at remembering the rules/reading dog body language sometimes . She's as committed to me about fostering and puts up with me nagging/lecturing(!) over an about creating good, calm behaviours and not reinforcing bad ones.
Some dogs are extremely forgiving but many put up with kids rather than liking them. Or kids innocently create I'll mannered dogs.
I place fosters as well as would never put one into a house with young kids without there having been a lot of meets with the dog/family and understanding on both sides as to what they are getting into. Because the worse thing that could happen (from the dogs point) is that it beets returned. No one wants a fox going from home to home.
You are right - he is an overgrown puppy and you need to be getting ground rules in place with him and the kids now.
He's going to be naturally bouncy and exuberant for some time. He's going to be be mouthy, he's going to try and nick food. He's a golden retriever puppy. It's the breed and his age. He needs a couple of hours off lead walks a day (or training equivalent) if you want him to be happy and sane.
The pack/pecking order theory of dog behaviour has been pretty much debunked. With the toast stealing he just read the room - so an inattentive person, and opportunity and grabbed it. I bet you had your attention fully on him when you were with him the day before - ware of him and attuned to his body language.
If it's a good rescue they will have a stable/list of trainers they use - this should be your starting point.
It's quite easy to train no jumping, leave it and other manners - using positive reinforcement. But I can't overstate the need for consistency enough.
All the training in the world is negated by say a 5!year old (quite naturally) wanting to dog to jump and play - the trouble is the dog doesn't know that this is conditions and just thinks it's fine/a good thing because it gets attention. The trouble is you can't turn that behaviour on and off like a switch.
Even DD who absolutely knows better will want to rough and tumble with a foster puppy and then whine/complain when the dog carries on that energy after she's had enough.
The constant refrain I'm my house is her "x os being annoying" me "you created that behaviour/energy"
Also along the lines why doesn't she/he do it tk you. It's not because I'm the pack leader - is because I am quiet and calm and confident and do t wind dogs up - because I'm a grown up and capable of responsibility and consequences
It's a lot to ask of kids.
They can be wonderful family dogs but they are not for the totally faint hearted. They are gentle and friendly but they are of the working dog l breeds and do best when their brains and bodies are given plenty to do.
He's a bit of blank canvass - and with the right support and long term training support could be a fabulous pet. But you do need to think really, really carefully about the commitment.
They are big dogs. Sweet ones, but it's a lot of dog if they are poorly trained.