There is a product called Bio-clean that is normally used for carpet cleaning to take the pee smell out,there is also another one used for pet pee smells that you can get at pet stores that is the same,I just don't know the name.
the best idea is to partially fill a bucket with this and soak the training pants for a day and then just toss both the pants and the bio-clean into the washer.
then a couple of rinses should do the trick.
I hope this helps.
My experience is that some fabrics hold odors more than other fabrics. The sooner you can get the wet clothes rinsed off, the better, so the odor doesn't have time to sit and absorb into the fabric. Even if all you can do is throw the undies in the sink and rinse them off, that's better than nothing.
Chemically, urine and vinegar are both acids; baking soda is a base. Using baking soda against urine odors will work better than vinegar since using a base against an acid neutralizes it.
I usually first rinse my kids' clothing that has urine in it, then I run it through a regular wash cycle and add some baking soda. I run the clothing through a rinse cycle before washing to get as much urine out as possible, so the baking soda doesn't have so much odor to neutralize.
I still have some clothing that I just can't get the odors out of, but this method has helped reduce the problem a LOT.