Personally, I believe all Children will be Raptured (I'm not sure if this can be proven in Scripture, I base this belief on The Lord being a just God), however, I also believe when that child becomes "of Age" they will be accountable to Christ. They will have to make the choice to choose Him. We don't make it to Heaven because our family chose to Receive Jesus, but because we did!
I say this with a heavy heart, because I have a twenty year old daughter who has been told the Truth of the Gospel on numerous occasions, she's not living the life of a believer. I will pray until my death for all my family to be saved.
In our earthly bodies living this life, we are saddened, overwhelmed even "panicy" for our loved ones to be saved. When we are in Heaven, if our loved ones are not there, we will understand why they are not and accept it. As much as we love them on Earth, I believe we will Love Jesus through and through and hate sin to the point, we won't want any non-believer or sin in His presence or ours, even if that includes our own family ( I feel like crying as I write this). But, He'll give us the grace, peace and security in knowing He was and is loving and just!
As much as I don't like to say this with unsaved family members still on this earth, still, Jesus is first in my life. Come quickly, Lord Jesus.