Children can not only leave their lots, they can also meet older Sims who are visiting their own lot, and attend wedding parties held on the lot they live on.
They can only escape their lots under extremely contrived circumstances. They can't call taxis, walk anywhere, or drive, making it extremely difficult to escape, and thus meet anyone in a controlled fashion. While it is possible for them to attend wedding parties on their OWN lots, as they were not "invited", they will not arrive in formal.