What is the ruling on saying "a divine slumber" when seeing a sleeping child?

There is no Sharia impediment to saying "may you have a blessed sleep from Allah." This saying is understood either as a prayer for the child to have a quiet and beneficial sleep, or as a statement about a blessing from Allah through their effortless sleep. Attributing the blessing to Allah here is in the context of divine giving and sustenance, not an affirmation of the attribute of sleep for Allah the Exalted, who is far removed from it, as stated in Ayatul Kursi and the noble Hadith: "Indeed, Allah عز وجل does not sleep, and it does not behoove Him to sleep."