Plumbing – Bad smell in down stairs bathroom

The two most common sources I've seen are dried out traps and a failing wax seal under the toilet. A failing wax seal may also result in water damage appearing from beneath the toilet. The dried out traps are simple to fix, just run a little water into each drain every few months.

For a, more rare, problem at a PVC joint, you'll need to find the offending joint first. Look for signs of water damage, or you may get lucky putting some soapy water over each joint and flushing a toilet.

Edit: here's a good description of how to replace the wax seal under the toilet: How do I fix a leaky toilet that only leaks from underneath?

Being a landlord I have tenants that just don't like to clean/scrub toilets weekly... try once or twice a year... however they do sometimes call up and say my toilet smells when I flush it. Every single time I have had this the solution has been the same with tenants... clean it with toilet bowl cleaner including under the RIM and clean it about 5 or 6 times as so much stuff is stuck everywhere... even change cleansers every other week.. but not in the same day (don't mix chemicals)...

Another possibility is the kid peed on the walls.. wash them.

Another possibility is ... are there warm air floor ducts near the toilet... maybe the kid pees in them for fun??? lift the covers and have a look... I did find one grill that was covered in pee spray... in a guest bathroom... once upon a time.

Another possibility there is a bad seal or no seal under the toilet... there is supposed to be a foam or wax ring under the toilet that keeps the air and water in the sewer system separate from the air and water in your house. I have had one instance in 20 years of a urine smell that happens every time you flush that cleansers did not solve... it turned out be a foam ring that was not wide enough to fill the gap between the floor and the bottom of the toilet and every time you flush - air from the sewer passes by after the water has passed by. Strangely there never were any water leaks that anyone ever noticed.

THe only way to check this is to remove the toilet by unscrewing the floor bolts, lifting and looking to see what is underneath... before you do this you might want to buy 3 regular wax rings and 3 extra thick wax rings... so you can choose which you want after you lift... if the rings are foam you might notice how one end might be squished and the other end appears full height... the full height is your clue that air might be getting through... Personally I don't care for the foam rings (hardware stores charge more for these than wax.. but it is harder to determine if you have a good fit.. with wax... it all just squishes out if it's too much and you get a good seal.

The last and least likely idea is that there is an issue with the vent stack for each toilet. Don't know the reputation or experience of your builder or the care with which city inspectors review builder work before signing off... but... you might want to check if there is a clear path from the stack inside the wall behind the toilet to the roof. Start with a quick look at each roof vent (the little chimney sticking out through the shingles) and see if you can see down it.. or is it plugged with a bird nest or some dead critter?