Remember your "infants" will be a year old when starting care so likely standing and cruising at least. Mostly they will just need an area where their toys can be and the older children need a reminder to stay away from the baby area - ie easier to teach the big kids to stay out of the baby's way than the other way around. Make an area for the older children with their big toys - like small lego or make those toys that only come out when the babies are napping.

Also since we have full day kindergarten in Ontario kids are only in care from age 1 - 3 so it was easier for me to remove toys meant for older kids and make the room pretty much safe for all ages as in nothing babies could choke on. I stay near the littlest ones but everyone has free reign of the play room.

There are play yards you can buy at places like ToysRus that are 6 or more sections you create a large safe play area with. They do take up quite a bit of space but will give you that secure feeling you are wanting.