If my DP had brought two children who weren't mine, to my house, for a time period of days and nights rather than hours, and had given me zero information on how to deal with them (or not deal with them), then yeah, it'd feel like she was forcing me to act as a parent.
What's he supposed to do? Ignore children messing up his house? He's a bachelor for chrissakes, he's not used to having children running amok under his feet 24 hours a day in his own home. He's probably not sleeping well either.
That's not what's happened here. They've both decided to go on holiday so they can all spend time together presumably, not so her children can run amok and she can offload some of the responsibility on another adult.
It's almost like none of you are men, have no idea how men think, and assume that everyone (man and woman) knows all the ins and outs of who is responsible for what when it comes to children
There's nothing to work out at this stage. They're HER children. She doesn't need to outline responsibilities, because the responsibilities are hers.
But no, never mind. Mumsnet obviously knows best, and he's a horrible person and you should get rid of him because he doesn't understand automatically how parental responsibility is assigned in this situation.
Do you reassign parental responsibility if you take your children anywhere in the company of other adults - to visit their house, on a day out or a trip away? No, why would you?
An adult is entitled to ask a child not to splash his £50 jeans, but the same adult is surely also aware if they're getting too frequently grumpy with the people they're trying to forge a bond with? Whether they are used to children or not.
Surely they should be mature enough to step away and defer to the parent rather than being moody with them. Especially when he is fully aware one has special needs.
If he thinks she doesn't discipline them enough, then that is an incompatibility that can either be worked on or they might decide is a deal-breaker.
It might have been unwise to go on holiday so soon, and it's understandable he'll get irritated sometimes. He had no idea what to expect, but similarly, she had no idea he would be unable to hide his impatience.
It's all a learning curve, she didn't need to hand him a manual, he's there to spend time with them.