Sandboxes can be unsanitary especially public sandboxes at public parks or any sandboxes in public areas.
Sandboxes that are left uncovered can attract animals that can poop and pee in the sandbox which then can make it unsanitary for the child to play in.
The children when young may also put sand in their mouths which if contaminated with the animal poop or pee could make the child sick.
But sandboxes at home or those sandboxes that are kept covered when not in use are okay and the child can play in the sandbox without getting sick.
Sandboxes do not have to be covered but it's recommended to cover sandboxes when not in use.
Covering the sandboxes keeps out animals which can use the sandbox as a toilet for peeing and pooping which can cause health issues if the kid puts the sand in their mouths which most kids will do.
My kids put sand in their mouths when they were little so I would always keep the sandbox covered when it's not in use.
If you buy a sandbox they will come with a cover to cover the sandbox up.
Kids who are around 12 to 18 months old take real easily to playing in a sandbox and they love to play with sand and pour the sand in and out of things including toy dump trucks etc.
Sand play is really a good play for kids and they may like to play in the sandbox until around 6 to 8 years old.
My kids played in the sandbox as early as age 10 months which is when I started them playing in it and they loved it.
You can make a sandbox and then put a cover on it when not in use to keep out rain and keep any cats or dogs out of it.
You don't want any animals pooping or peeing in the sandbox because the kids may put sand in their mouths.