For safety, your child should be taught to float on his back first. Next comes front crawl (or ‘freestyle’ in some countries).
Front crawl is best stroke for safety because it is faster and stronger. If you want to get out of danger (whether to exit a rip tide or to dodge someone coming down a slide) you want to do it quickly and exerting as little energy as possible. That’s front crawl.
Breaststroke is more difficult because you need to coordinate different arm and leg movements. Can you or your child simultaneously pat your head and rub your tummy? Breaststroke is a bit like that - doable but tricky until you get the hang of it.
Any stroke that keeps your head out of the water will tire you more quickly than a stroke with your head in the water. Getting tired in the water is dangerous. So, for safety, we focus on being comfortable with your head in the water. It reduces strain on your neck and reduces panic if the swimmer is splashed or hit by a big wave.
Breaststroke may appear similar treading water. It isn’t. Proper treading water takes very little energy. Breaststroke takes quite a lot of energy, even if you stay in one place. For safety, you don’t want to run out of energy in the water.
I STRONGLY advise you to avoid any swim school that teaches breaststroke first and to do whatever you can to get your child comfortable with his head under the water. Try in the bath tub and shower at home, splashing in the pool, making faces underwater, picking things up from the bottom. The pool will probably have buckets with holes in the bottom (or similar devices) so you can pour water over each other to acclimatize new swimmers to this.