Thank you for the question. It is not that limbo was rescinded, but that it was never taught as an "official" church doctrine. Rather it was an attempt to bring together two doctrines: the necessity of baptism for salvation and the mercy/justice of God. We really do not have an answer altho the pope did make an interesting reflection in Evangelium Vitae # 99. Have you read it? It applies to babies who are aborted, but would have a broader application.
Fr. Phil Bloom
P.S. The fate of unbaptized children is something I think about myself. I wrote a short story about unbaptized babies you might like: Marina and the Babies
Another letter about Limbo