So your child's room is such a mess there is only a rumor of a floor and the cat has been lost in there for several days, probably dining off bits of leftover forgotten snacks (well, at least they won't go all moldy and attract bugs).
Shut the door and walk away. Take deep breaths and set aside the urge to cling to your child's legs, begging to be let in to clean it up! Rejoice in the fact that it's that person's room, to do with what they will. If they don't like it the way it is, they will do something about it – maybe even ask for help in cleaning it up! (dream on...)
When you do break down and end up standing forlorn outside your child's door, armed with feather duster and vacuum cleaner, acknowledge that it is not your business, and that these lapses in your demeanor are due to your own bad theories that you are weeding through and working to change, and ask for their understanding and forgiveness when this happens and thank them for their helping you to realize where your hang-ups are and how much you appreciate their patience and good grace in helping you, yet again, to examine this subject.
Then take a look at your own unfinished projects laying around, that you are so willing to procrastinate upon further that you would rather clean your child's room. Dig into your own stuff! Direct that frenzy of energy there! Or go groom the dog or read a good book in a lovely hot scented bath, or grab your partner and head to the passion pit, or find something else that you really enjoy and want to do in the moment.
Even when grandparents visit, pulling the door closed on a child's room and protecting their privacy is the child's right. Your child and their friends will happily disappear into that room – they don't care about the mess. They don't see it; they only see the cool stuff and the wonderful possibilities.