Two-Year-Olds and Turntables...

That is a tricky situation, can you place a chair in front of it so it is hidden behind the chairs' high back?

I took a different approach, which has worked (so far).

We have a 22 month old, and I bought him an old Kenwood direct drive that he can play with all he wants (not plugged in), it's just sitting on the floor in his play space and he has his own little library of records that he can chew, dance on, or spin - it's up to him. He gets excited when I bring him a new record too! Once he's old enough I'll see if the table will still work so he can really use it, but for now, it definitely keeps the heat off mine.

A very none politically correct idea is
It may be your grandchild but it is your offspring's child, hence if they wish to visit and have you be relaxed and enjoy it, it is their responsibility to either pay attention to their child or not stay to long.
A visit is meant to be pleasant not unnerving
Having no grandchildren yet I can say this, when I do I may regret it.

Call Dr. Laura. I will save you the call and say that you have failed in parenting. When I was a child NO meant NO!!! A cattle prod might just be the trick though. Maybe a barbed wire or small electric fence might work also.

Fortunately my kids are old enough that they know that to touch my stereo guarantees a serious readjustment of their VTF (especially when they later try to sit).
If I had a baby or toddler to worry about, I'd put something big and/or heavy on top of a tablecloth on top of the tt. Out of sight, out of mind, out of harm's way.
Uru, kudos for bucking the pc trend. In my experience however, even the best-behaved kids are prone to accidents or unprecedented dares. With a deep-four-figure cartridge that no child would recognize as pricey and frangible, for my own peace of mind I'd opt for removing (disguising) the temptation.
Save the cattle prods for cable burn-in, amigos!!
cheers apo