This is off topic, but unconditional love is something I personally have been looking for. To me it would be, that someone loves you without wanting to use you, someone who really loves you because of you. I disagree with you that a child couldn't give unconditional love, it could do that when it's not forced and when the relationship with the parent(s) is oké.
I think only your own children are capable of that, but only if you treat your children as children, as human beings. We as parents, shouldn't force our children into something that would make them unhappy. We should teach them what we know. But not force them, we should also listen to what they have to say and respect their opinions, even if they are younger. We should try to understand our children.
When you have a better relationship with your children, I think they are probably the only ones to love unconditionally. If I would buy my children something or cook something for them, they would never falsely accuse me of "wanting something from them", that's just one example. I think the relationship with your own children is one of the 'safest' relationships possible, but only if you as a parent give your child what your child needs.
Children are humans, they have feelings and opinions, they want to be heard, they want to be taken seriously too. Even though they don't understand many things yet, we should treat them as humans instead of only ordering them around and forcing them into decisions that would only hurt them. They don't necesarily have to agree with us.
I think certain cases, children can comfort their parents, if the relationship between them is oké. It doesn't mean that the children can't have their own life. They should have their own lifes and their own choices.
Unconditional love is something I've never found with anyone, only finding that now with my own children.
But every situation is different of course. And everyone has a different opinion. This is just my opinion about parents and children and unconditional love.