That all depends on the agreement you have with the parents to transport their child off site, and if you have their permission to take their child via van, as well as other factors that would make you subject to the laws regarding transporting students, as it applies to your state. In my state, your van must meet certain safety criteria, and the driver must have a CDL license. Personally, if I owned a day care, I would not want to take young children off site. You need a safe student/teacher ratio, and a staff you can completely depend on to keep the children safe in their care at all times. In addition, you may want to check with your insurance company to make sure you are covered for anything that may come up once you are off site with the children.
(do yourself a favor, and create a small play yard of your own so you can remain on site!)