I answered to your same question not long ago

Believe me, DS-156 is the same form that is used when you file for other types of non-immigrant visas, like tourist visas, and every applicant needs one. Even if they are underage, they need to fill their form with their OWN info.
It does not make any sense to fill out the mother's information if the kids will need a separate visa (K-2) for each. DS-156K it's only for the mother, and of course, you need to write that the children will be acompanying her on the trip.
If you feel at more ease filling out 2 different versions, it's fine.. but trust me, each applicant needs one DS-156 with their own information.