Present perfect allows "wider" timespan which is shown in your examples. I haven't seen her today it means that a whole (or half, doesn't matter) day has passed, and you still haven't seen her.
But in the sentence above, They didn't go to school today, the action of going to school happens at a fixed daytime, and it is a one-time action. This makes it a perfect match for Past Simple.
"Onlar bugün okula gitmediler." Translation: They did not go to school today.
Burada "to the school" olarak kullanım yapılmaması neden? Yani "the" neden kullanılmamış. Bunu tam olarak anlayamadım.
Okul - school - (noun)
Okul + "-a" dative case noun suffix meaning "to" school not "the" school.
"The school was closed today" - Bugün okul kapanmış. Çocuklar okula gitmediler. - "The children did not go to school."
Do you understand it now? Şimdi anladım mı?
Lütfen, hata yapdıysam duzelt.