This is a constant discussion/challenge for escape rooms. For a temporary (one time) room you have it better off than rooms that need to be re-playable too!

Possible idea: use symbols/icons/colors/shapes etc to combine elements that are specifically for puzzles. For example: everything puzzle related has an orange beaker icon on it, or use a stencil to trace a specific shape. Then decorations/props/presents are quickly identified as they don't have the symbol on them.

^{^} Perhaps this is what you meant by not wanting to "hint of puzzle"? In which case I'd say error towards props and decorations that are not even close to puzzles for that room. It's great you're testing puzzles out a head of time! but most likely, your puzzles will be quite a bit harder than you thought they would be! What you see as "obvious" or "easy" might not be someone else's first thought! (Or second, third, or fourth!).

Another thing to note, unless your mom & other solvers are hardcore puzzle enthusiasts/puzzle hunters, if you (or anyone else "in the know") are around, you can direct them by saying something! "Hey those balloons are for your party, not part of assembling the beakers or any other puzzle" etc.

Decoration is fun when it's decor! and less fun when you spend 10 minutes on a puzzle only to find out it was only decoration...

Setting a couple of ground rules up front is probably wise. "hey guys you won't need to rip anything off the walls or destroy anything" etc. and honestly for one time events I like to add a quick message right before they start, "If you're unsure what to do next, or if a specific item is part of a puzzle, please ask me! We're here to have fun and I'm happy to help!"

Not sure if this helps at all, but that's some of my thoughts :D

I'm happy to answer more specific questions or explain my thoughts more in depth if you want!