Its really a tough call since its a 7 year old child you can just say "you know beta everyone has to pay for his amaals but we can always pray for our loved ones that they start praying and remember ALLAH.ALLAH listens to children and when you will pray your dad will start praying" Or "We can always pray to ALLAH about everything and you can always pray to ALLAH about your dad "AYE ALLAH please help my dad so he can remember you" And take the conversation to praying so they can ask less questions.
PS.Wife can talk to dad about that