Imaam al-Qurtubi notes in his Tafsir:
وأما تسليم الكبير على الصغير فروى أشعث عن الحسن أنه كان لا يرى التسليم على الصبيان ؛ قال : لأن الرد فرض والصبي لا يلزمه الرد فلا ينبغي أن يسلم عليهم
أشعث narrated from al-Hasan that he did not think that one should say salaam to children. He said: This is because to return the Salaam is obligatory while the child is not obligated to return the greeting so it is not fitting that one says salaam to children.
This was Qiyaas from al-Hasan. Despite his high and noble status, the Qiyaas was incorrect.
Imaam al-Qurtubi notes:
وقال أكثر العلماء : التسليم عليهم أفضل من تركه
The majority of scholars said that saying Salaam to children is better than not doing so.
And we have evidence in the Sunnah to support this correct view
I was walking with Thaabit al-Bunaani that he happened to pass by children and he greeted them. And Thaabit reported that he walked with Anas and he happened to pass by children and he greeted them. and Anas reported that he walked with the Messenger of Allaah (sallallaahu alayhi wa sallam) and he happened to pass by children and he greeted them.
Sahih Muslim Hadith No. 2168