How do I know what nut to get for training wheels? We got a dynacraft kids bike for my daughter off facebook market, but need nuts to attach the training wheels. The ones we have don't seem to be threaded right or something (won't tighten all the way without becoming loose again, like the threads are slipping).
I've looked everywhere online but can't find anything saying what kind of nut it should be. How can I go about figuring it out? Dynacraft won't let me contact support without a serial # and I see no serial # on the bike anywhere.
The other answer is functional, but buying tools to check threads is going way beyond what someone needs who just wants to put training wheels on a bike.  Assuming you are in a North American country, head out to the nearest hardware store (Ace, True Value, etc.) or home improvement store (Lowes, Home Depot, etc.) and bring the training wheels with you.  Its possible that the threads are destroyed on either the nut or bolt.  The employees at these stores are always helpful and can find exactly what you need, or sometimes see issues that you can't.  If employees aren't there or not helpful, there are usually nuts and bolts in the hardware section you can use to test out different sizes for fit.