Would you sacrifice your child to gain immortality?
They'd more than likely say "no".
Follow up with: But this is exactly what you're doing. You hope to live forever, and are willing to cut off your own children forever in order to gain that prize.
If they say they worship God because he's their creator or because they love Him:
Why would you worship a deity that promises to kill your child?
Isn't that what the Ba'el worshipers were accused of?
This deity doesn't sound like a god of love and peace at all. He sounds like one of chaos and destruction.
Why do you love this God?
They may say "because he made me".
To which you follow it up with:
But he also threatens to kill you unless you worship him. Is that something a loving parent does?