is thuja plicata safe to plant as a hedge with children and animals

It's not on the poisonous plants list I have - so I would think it's OK.

Why would they want to eat the leaves, just as a matter of interest?

Yes Barbara, something that's always been a bit lost on me :0)

Kids do, sometimes. I have a problem with conifers generally if I brush against them, they bring my skin up in red weals which can be itchy, but I think that's just a contact allergy, not because they're poisonous. Yew's the only one I know about as poisonous.

Kids need to be taught not to eat leaves, berries etc. Get them to help with the planting and explain that the plants are for looking at! Yews only have poisonous seeds, even the red fruit is OK to eat just not the seed inside ( I still wouldn't like to try one!!)

Yew leaves are poisonous to animals. You are right about the red fruit, though. Henry ate a whole lot of berries and had to be rushed to the vet's, where they looked it all up. He hadn't chewed them, or eaten any leaves, so he was fine.

That's why farmers won't have yew trees by fields - if horses or cows (or dogs!!) ate yew leaves, they'd die, apparently.

Yew trees ARE poisonous. Every part from root, bark, leaves and seed. The only part not poisonous is the red fleshy fruit (ariel), which tasts very sweet, but you must not crunch, crack or swallow the seeds.
Yew (Taxus) poisoning can be fatal and is a painful experience.
It doesn't take much so take care!
Yew trees are often found in church yards (cattle don't graze there).Bows are made from them as well as from Ash. A Yew bow can be bent in a U without snapping.
This is getting long winded so I'd better bow (groan) out now.