Since you plan to relocate them to the US, N-600K is not appropriate. N-600K is for children who intend to remain residing abroad. The process requires them to enter the US on a visitor visa or other nonimmigrant status, to attend the interview and oath. If they have immigrant intent, they will likely be denied a US visitor visa or denied entry to the US on visitor status.
Instead, the path to US citizenship for children who intend to relocate to the US is to become US permanent residents (i.e. green card holders), and get automatic US citizenship through INA 320, by living in the US with a US citizen parent (or living as a military dependent of a US citizen parent stationed abroad) while under 18. They used to have green cards, but their residence would likely be considered to have been abandoned by now. The fact that their green cards expired has no bearing on whether their residence is abandoned. And length of absence is only one of many factors in determining abandonment of residence. However, the very long absence here (if you naturalized in 2015 and they have not lived in the US since then, so at least 7 years of not living in the US) makes it very likely that they would be considered to have abandoned residence. And thus you would have to petition them to immigrate (by filing I-130) again. When they enter the US with their immigrant visa, they immediately become US permanent residents (i.e. green card holders), and also (if they are under 18 and living with you) immediately become US citizens under INA 320. Thereafter, they can apply for US passports at any time. They can also reopen the denied N-600 to apply for a Certificate of Citizenship at any time if they want, though it serves little use that their US passport doesn't already serve as.
