Depends on where I think the older child got it from and how likely it is to be spread. Even if other kids at the daycare come down with whatever there is always a chance that it didn't happen at daycare and instead out in the real world.

I probably would have done what you did and accepted the younger child but at the first sign of problems sent him home instead of taking a wait and see approach given the older sibling.

Just because one child in a family has something it doesn't mean it will spread through them all and there is a better chance the younger child won't get it if they are with you instead of stays home with the sick child and is around them all day then they both come back only for the younger one to get really sick at daycare now exposing everyone. Limiting the younger one's exposure by sending him to you may actually save your daycare group from all getting it.

The fact the group was already exposed to it once also shows how we can not realy prevent anything. If they get it now it will be because they were already exposed.