Can we believe, as Catholics, in infant damnation? Is it still a valid theological opinion for those who take the second council of Lyons and Pope Gregory X seriously?
I’m not sure what your Gregory X reference is, but your Second Council of Lyons reference is clearly to a document that was actually preconciliar. It was a transcription by the eastern emperor of the text of a profession of faith that was proposed to him by the Pope. Although the document was read before the Council Fathers, it was not written at the Council, was not promulgated by the Council, or even discussed by the Council. Thus, the dogmatic impact of the document is questionable at best.
Even if you were to take the document as absolute dogma, it does not consign infants to everlasting damnation. It only talks about what happens immediately after dying in original sin only (as an unbaptized infant), i.e. descent into hell to receive different punishments than those who die in mortal sin. But the same document goes on to talk about the final judgment where everyone will be judged according to their works. You have to remember that there is hell as in Hades, and hell as in Gehenna. Gehenna is the final state of the damned. The document in question refers to those dying in original sin descending into Hades.
On the other hand there is Jesus, who said regarding little children that “their angels always behold the face of my Father who is in heaven.” For us to enter the kingdom of heaven, we must become like those little children.
By the way, when I read the Catechism, the only way that I see that one gets eternally damned is to die in mortal sin. Obviously, infants can’t commit mortal sin, and, in the end, heaven or hell are the only options.
So, to answer your question, I’m not aware of any direct prohibition against your holding such a belief. I suppose if you represent this belief as something that Catholics ought to believe, you may turn some away from the faith due to the doctrine’s utter repugnancy. But that wouldn’t be mortal sin in you, I don’t think, because you actually believe in the doctrine. How you can be happy or find motivation for serving God other than fear when you believe such a thing is something I can’t answer.