Thanks for curating!
It's indeed really hard to shield kids with access to technology, there are some tools that can help like parental controls settings and apps.
Letting they access only trustworthy sites and streaming platforms like Disney and discovery kids is a safe bet too (I wouldn't trust YouTube kids for that).
It's indeed better to not look behind curtains sometimes. You sound like an awesome parent/grandparent that is giving active care and support to your kids so I'm sure they are good even in case something gets through!