If they were clear that they literally needed a place to stay vs. coming to visit you while also attending a wedding, then I think their transgressions are limited to the disrespect they showed to your home. As a child, even if I had woken up before anyone else in the house, I knew that it was important to be quiet and also that I wasn't to move things around or mess up my surroundings. My mom never specifically told me, "if you get up early, you should do x, y, and z" but she never had to. I was taught the general rule of being respectful and considerate of others. If I had messed up somehow, my parents would have cleaned it up (or had me do it) and apologized (and then had me apologize too). If they couldn't trust their kids to do that, they should have had a parent up with them to supervise and also cleaned up whatever mess what made.
I agree. Your friend was upfront about needing somewhere to stay to attend the wedding.The rest, however, was really rude.
If you [generic you] have a child who wets the bed you need to bring a mattress protector unless you have checked in advance with your host that they have one (there may be exceptions if your host also has young children and you are 100% certain that they have a spare) And I think you should be asking whether you need to bring other bedding, too. And at least offer to deal with the resulting laundry.
Same thing with the mess. As parents, they ought to have done their best to prevent their kids messing up your house, and have cleared up after themselves (again, if you offer to do this, that's fine, but it's their responsibility, not yours)
The nappies - I think it's odd to just leave these in the bedroom. Thinking of friends with babies who have been visiting me or other friends, they tend to ask (a) where they can change nappies and (b) where to put the dirty nappies - i.e 'is there a bin in bathroom, or should I put it straight outside?'
And there is no excuse whatsoever for their lack of thanks or even response to your text.
Given that they know from the start they were coming for accommodation rather than socialising they definitely should have either brought a gift (flowers, chocolate, wine) or sent something afterwards.
If it were me, I would not be inviting these people back to my home.