There are several companies that make wooden or plastic trains that are all made to run on the same track - see link below. If it's those trains, the gauge is about HO gauge , but the individual cars and engines are quite a bit undersized regarding height and length. Most likely N scale decals would be the best bet.
https://www.amazon.com/Wooden-Train-Track-Piece-Pack/dp/B005AW85YG
You don't give clue as to what size your boy's trains are. Wooden trains are in all different sizes., from huge to tiny, all of which are pretty much out of proportion, with wheels and stuff.
If you have a scale ruler, you can do the math and figure out what the closest model railroad size is that you have, if you feel it's really important.
I'd just go by eye, if looks close, use it, no matter what the scale. I'm sure your boys won't care.
Mike.
My You Tube
The wooden trains aren't made to any specific scale, so you're not really going to find an answer to this question.
I'd say they're probably somewhere between N and HO, but not to any scale proportion - so N scale decals might work - HO might be too large to fit.
Chris van der Heide
My Algoma Central Railway Modeling Blog
My 5 year old twin boys have plenty of wooden trains. Some could use repainting. Would like to know what size decals would work best. Google search hasn't given me any delineate scale size. My thoughts are HO scale but value the info of the forum. Has anyone here done this also or will I be the test pilot here?