Is this sentence correct if I describe a group of children?
Children 6-7 years olds.
You can say, "Children 6-7 years old", or "6-7-year-old children".
When used as a "normal" adjective, a number and unit are combined with a hyphen and the unit is given in the singular. "A 10-foot pole", "a 6-year-old child", "the 12-parsec Kessel run", etc.
When used as a predicate adjective, the number and unit are not hyphenated and the unit is given as a plural if the number is not 1. "She is 6 years old." "The house is one floor." Etc.
Depends on the context, if it's a group of children 5-7 years of age, the correct sentence would be
6-7 year olds
is used to describe multiple children
such as in a sentence:
it's a group of 6-7 year olds
6-7 years old
is used to describe a child that is 6-7 years of age.
The children word is not really needed here in the sentence, it doesn't make very much sense.
unless in a context of for example:
Only children 6-7 years old allowed.