Contrary to what is being taught today by many, the teaching of Limbo was not merely speculation: it is a doctrinal teaching of the Church.
It is true that the dogma has never been defined, but that does not mean it is merely speculation. Limbo has been taught by the ordinary - and extraordinary (i.e. infallible) magisterium over the past 2000 years. The only question about Limbo that needs clarification by the Church, is whether or not the unbaptized child suffers to actual fires of hell, as some have taught, or whether they are in a state of natural happiness, as other have taught.
The following article gives many of the magesterial quotes regarding Limbo.
thecatholicfaith.blogspot.com/
Another good source is the Catholic encyclopedia at
newadvent.org:
newadvent.org/cathen/09256a.htm