No, we can only know ‘for certain’ that God is merciful.
That is why if the unbaptized children are in heaven it would surprise no one.
But while we can say, “I strongly believe the children are in heaven”, we must stop short of saying, “I am certain that they are in heaven” IF by that we are expressing our own personal ‘belief’ and presenting it as truth.
Agreed, we cannot circumvent the Church’s teaching on such matters, but the Church has no set teaching or dogma concerning this topic. I will not deny the authority that belongs to the Church, and you are correct that careful wording should be used when expressing ones opinions on a matter that is considered ‘open for interpretation’, when the Church hasn’t decided on something like this (because, indeed, there is a potential to misrepresent the teachings of the Church).
That said, I do not deny that Baptism is saving, I understand and agree with this teaching 100%. I also do not deny what Jesus said concerning the children, and that was that the Kingdom of Heaven belonged to such as them. So, while I respect the authority of the Church on these matters, it is difficult for me personally to ignore the Word of God, and say, “I do not know for certain.” when I am asked to answer what I believe. Yes, I tell them what the Church teaches (which is basically ‘we do not know’), but then I tell what I have come to understand, which is reasonable since we are without any real answer.
What I have been saying is that we all should know, and we have a responsibility to know what Jesus revealed to us concerning this. For, if we do not know for certain, then how can we be certain that Martyrs are in heaven? Jesus revealed to us that whoever dies for his sake is saved, so then why, when Jesus reveals to us that the Kingdom of God belongs to the little children, is it a hard thing to grasp?
Now, saying “unbaptized children are in heaven” does not go against saying 'unbaptized people cannot get into heaven.", because we know for certain there is another type of baptism, and those who partake in that baptism are saved. But one might say that martyrs were murdered because of their belief, but this was not true of the thief on the cross who was saved because he repented of his sin, and who was unable to be baptized in water.
So when we take these things into consideration and we read from the Scripture what Jesus said concerning the Children we CAN say, “I know for certain they are in heaven.”, and we can say this without any offense to God, because we are not judging that person anymore then we judge a Saint or a Martyr. And if we say we don’t know for certain, then we are wrong in saying we are certain that anyone is in heaven. For if we doubt the Word of God on one thing, then how can we go on without doubting on another? If we entrust someone to Gods mercy (and we say we do), then we trust that God has saved them, because he is merciful, and God does not condemn the innocent. Then, therefore, if we turn around and say, ‘but we do not know there fate’, where then is our trust in God?