The only issue you may encounter with a ‘normal’ wrench is that the flats may be too thick to fit on the flats of the pedal spindle depending on the design. If it’s an old wrench, you can grind them down to fit but that’s a decent amount of work even with a bench grinder (and I’m guessing you don’t just have one sitting nearby). If it can wait a few days, a proper pedal wrench will make your life much easier. If nothing else, it will be longer giving you more leverage for the inevitably over-tightened pedals you’ll encounter.
FWIW, both of my sons learned to coast on a Strider by age 2 and were pedaling on a two wheeler at age 3. I am a huge fan of the no-pedals technique and recommend it to anyone learning to ride a bike or even just reacquaint themselves with one.