Are they happy in Heaven knowing their child is suffering eternally in Hell?
You've already asked this, and had it answered. Heaven is a state of bliss. There are no tears, no regrets, no unhappy things.
If it was God's Will for a child to go unredeemed, by the time we get to Heaven we will be in perfect agreement with God's Will. We will nto wish things were otherwise. We will see God as He Is, and all will be Perfection.
SD
I think I answered that in another post, Ken.