No, you can’t. Since grouping is easy, why would you want to?

So I have a Sonos Beam, Play One SL's and the Sub all grouped together in the lounge. We've recently had a little boy and he likes gentle music to sooth him. Obviously I don't want to play it through the whole set up as it's a bit loud and has bass. But when I'm trying to calm him I don't really want to be splitting up groups and then needing to remake them. So just one speaker would be easier.

Sounds like you have then Bound, not Grouped. (Bound is how surrounds and subs are connected to a home theater speaker).

The answer is No. It is a common request to make it easier to un-bind and re-bind speakers.

Grouping and un-grouping is quick and easy, but that isn’t what you are really asking.

I think you mean you actually have the speakers bonded together to form a single room. “Group” in Sonos speaks means several room temporarily playing together in sync.

So no, there isn’t any easy way to quickly unbond speakers without having to go back through setup to get it back to a home theatre setup.  I would recommend getting a Sonos Roam for your needs.   Yes, it’s extra money, but it would be small and portable and your little one can even hold it and slobber all over it if he wants.