I think if the child is fine to participate I would let them come. The siblings I had, often one would be sick and the other fine. Same with my daughters where one won't get it.
I don't know if I like the comment about parents being lazy. Having the other child away is nice for the sick child as they get more 1:1 time when they need it most and the well child gets a break from the TV as mom/dad may be busy with the sick kid.