Selam. I need you to reply this question as if it doesn't contain a single mistake or unclarity in its description. If someone makes the intention of imagining the sentence < The almighty has a child> and he imagines that sentence as if he is seeing it in a book or letter. He imagines that sentence word by word. He doesn't imagine sounds or voices. He simply imagines that sentence. Level of sin? The only intention he has is imagining that sentence. < Yzeir is the son of God> Idols are intermediatery to God > In other words imagining those blasphemous sayings in the manner I described above.
Wa alaikumu As-Salamu wa Rahmatullahi wa Barakatuh.
What matters is your heart; as long as you don't believe in it, in shaa Allah it's fine.
I advice you to avoid such thoughts and make Dhikr (Subhanallah (Pure and Perfect is Allah, for He has no error whatsoever), Al-Hamdulilah (All praise is due to Allah, for all the blessings, such as the sight, the hearing and the mind, that He has given us), Allahu Akbar (Allah is the Greatest, and there's no one that can challenge Him in power, if all mankind and Jinn were to commit Kufr as much as the most mischievous of His slaves, that would not harm Him, and if all mankind and Jinn were to have Iman like that of the most righteous one of His slaves, that would not raise Him higher), and others)
Also read more Qur'an and make more voluntary deeds, for these in shaa Allah help strengthen your Iman, and remember:
Indeed, Allah is with the patient
Brother if you have any question you may ask, may Allah reward you with good