That's a form of the general question: Why does anyone suffer? Or more narrow: Why do the innocent suffer?
Then you seem to ask next: How could a child that did not suffer be ready for heaven? (as you worded it: "if this child only ascends to heaven, for eternity, how would the child know it is eternal bliss, if no bad exists in heaven"?)
Of course, obviously if they suffer here in a mortal body first, then they can know bliss is better than suffering from experience.
But that's not the whole answer. There is more. God is the One Who is able to know what we need, individually.
Individually: different souls needing different things to be completely ready. Some needing more, but some not needing as much, because they are more close to just-right already. Maybe some souls already really in a perfect place, even before birth.
But if not, He can do what it takes -- see 1rst Peter 3:18-4:6: souls can be given a chance after this mortal life to turn to Christ in faith, and be changed for the better, even after the mortal body has perished.
All of the above also answers the last question (if you were paying attention). To restate (in case you prefer): God is the one who knows just what the individual, unique soul needs, uniquely.
Could I suggest you are approaching Christianity wrong? It's not about proving it correct or incorrect by logic -- neither is possible. Instead, to try out Christianity in a real way, one would have to do the specific things Jesus taught we must do, the things He indicated are essential. There's not another way to find out in a conclusive way.