The smell is in my baby's room and only seems to be coming from there. It starts about 10-20 minutes into the wash cycle for my washing machine. All drains seem to drain fine. A plumber suggested that it could be a busted sewer pipe running under the floor at that point of the house. No crawl space home is on a slab. What do I do? And what could it be?

It might be the baby!(just joking). Are you sure that it's the washing machine? It might be your neibours' pipes problem.

Try find out exactly where the smell is coming from

I'm wondering if you might not have a open vent pipe in the wall.
Have this checked by a sharp plumber. You may require a smoke test to locate the source.
Sewer gas is harmful to your families health and the methane content makes it explosive. Since it's the babies room that's affected I would get on this ASAP. Good luck, Tom