As far as I know, no tarantula venom is lethal to humans. Most are the equivalent of a bee or wasp sting with some species having a bit more potent venom. A taranluta bite will usually cause some swelling, redness and pain in the area.
As a risk to your children, it is very low. The only danger being they get bitten in a very sensitive area (i.e. in the face near eyes: very rare) or if they are allergic to the venom as some are allergic to bee and wasp stings. Another more common risk: most tarantulas flick urticating hairs as a defense, which can be very irritating to the skin and troublesome if gotten in the eyes.
These are the worse case scenarios you can encounter with a tarantula. Up to you to choose if you want one in your household. I for one very much enjoy them as pets and would recommend them to anyone. In a household with young children I would caution to keep the tarantula out of reach or in a setup that children can't open by themselves.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!