Second, check if the clock has "delay" function. "Delay" is used more and more often in US tournaments.
Third, go to tournaments to find out which one you like. You don't need clock until your child begins to play in tournaments.
For beginners, I suggest: DGT North American Chess Clock
For advanced players: Chronos GX Touch Chess Clock
You can find them and other clocks in online stores. Compare prices. There is some difference.