I am strongly recommending not to buy any bike with SunRace twist-style shifters, which are found mostly on kids bikes. I have come across these multiple times and every time they are incredibly hard to shift. So hard that kids are unable to shift the bike at all. At first I thought the problem was related to cable friction or friction with the abutting rubber hand grip. However the problem is these shifters have a design flaw and do not provide sufficient cable pull leverage with the rear derailleur.
I have encountered these shifters multiple times now on the Diamond Back Tess 20 and 24 bike, which is a popular kids bike from Sports Authority, Dicks Sporting Goods, some REI stores, and other retailers.
I recently replaced the SunRace shifters with a Shimano Revo shifter with great success. The Revo shifters provide the necessary leverage to operate the derailleur and allow kids to use their gears. Call me for great pricing on the Revo shifter and installation.
I now must also include Microshift twist style shifters as well. Although these aere not as hard to shift as SunRace, they are still too difficult for most kids, or at the very least kids will shift a lot less and enjoy the bike and ride less from them.