"남자아이와 여자아이는 적습니다."
Translation:The boy and the girl write.
it's from the verb 적다. to conjugate in the non-past indicative, you take its stem 적 then add the declarative suffix 습니다 to form the formal polite speech level we are learning here.
**note that the declarative formal polite suffix has two forms, ㅂ니다 and 습니다. if the stem of the verb or adjective has no batchim (final consonant) or its batchim is ㄹ, ㅂ니다 is attached as suffix. if the stem does have a batchim, 습니다 is then used instead.