illness vs illnesses

Personally, I think you can use either.

I think you can drop the plural possessive on son and use illness, I'm not sure why but the double possessive doesn't work so well with the plural on son.

"I pray that my son' and daughter's illness be cured soon."

Unless, of course, you have more than one son and daughter.

I think you can drop the plural possessive on son and use illness, I'm not sure why but the double possessive doesn't work so well with the plural on son.
"I pray that my son' and daughter's illness be cured soon."
Unless, of course, you have more than one son and daughter.

I have not seen son' used in this way. Thanks.