Translation:The time with the children is valuable.
Because of the ending -en. For neuter (ett) words, the definite form is -et in the singular (barnet) and -en in the plural (barnen). But for en-words the -en is the definite in the singular.
Is it because "tiden är värdefull" is the main part of the sentence? Thus "värdefull" corresponds to the -en ending of "tiden"?