unscrew a rounded nut
You need &quot;locking pliers&quot; - Vice Grips™ is a trade name that has become a common name for these types of pliers. They will lock on to any portion of that nut, embossing some grooves in it, and allow you to turn the nut. You'll have to squeeze the pliers quite hard to get them to lock, but once they're locked, they'll hold just fine. You then unscrew, applying whatever turning force is necessary without worrying about holding the pliers on to the nut. Depending on how tight the nut is, you may have to release &amp; relock the pliers more than once as you run out of turning room.

With very few exceptions, all threads on a bike are standard right-hand threads (righty-tightey, lefty-loosey). The exceptions are the right-hand side pedal and crank arm. You won't have to worry about the crank arm on this bike, though, as it appears to have a 1-piece crank. There may be a couple of other left-hand threads, but they're on parts you won't ever encounter until you're deep into home bike maintenance - the kind of maintenance you'll never do on this bike because the kid will have outgrown it long before that maintenance is necessary.


As a side note, you're probably going to want to replace the nuts on the rear axle where the training wheels attach. This will help prevent the kid from scraping himself on the potentially sharp edge of the nut. You'll probably want an acorn nut (which will cover then end of the bolt with a smoothly rounded &quot;lid&quot;). You will not want to reuse whatever's left of the nut in your picture - it will have more sharp edges than the axle! When you get that nut off, take it to your local hardware store, and ask for a pair of acorn nuts - you'll need this bolt so you can be sure the threads match.