It is very easy to figure things out in retrospective yes?
So one must cover the mattress with something waterproof. A zip on soft plastic works well.
Or pads, we had quilted cotton pads. Now they have medical pads that also do the job.
Ok, think I would try the enzyme thingys , see how they work.
You could wet the area with lemon juice, crust the damp juice with salt, and hang the mattress in the sun until it dries. Not sure how or why this works but it is rather an old remedy and worked fairly well in my day.
I am sure there must be new products on the market as effective if not more so but the lemon juice, salt, and sunshine smelled so fresh and good after.
Unless you have a very rare child who stops peeing at night before they are seven I would always protect the mattress.
Children simply get overtired, or have a last minute drink before bedtime, and just cannot keep from peeing at night occasionally.
Best wishes and God bless