Mary was a good person and had a sinful nature like all of us.

[NotUrAvgGuy]

Catechism of the Catholic Church:
1261 As regards children who have died without Baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus' tenderness toward children which caused him to say: "Let the children come to me, do not hinder them,"63 allow us to hope that there is a way of salvation for children who have died without Baptism. All the more urgent is the Church's call not to prevent little children coming to Christ through the gift of holy Baptism.

Catechism of the Catholic Church - IntraText

Then there is the doctrinal mess over limbo: What's the Deal with Limbo?.

It's not official doctrine yet it is still considered a possibility. The Catholic church has changed it from being "common doctrine" to "a possibly theological hypothesis." Hmmm...Christ's church on earth cannot make up its mind on this and has even changed its mind. The whole subject seems very messy. I also don't see a clear understanding of the Bible's teaching of what happens to our souls when we die. The majority Biblical interpretation is that when a person dies on earth, if they are a believer their soul goes straight to heaven; if they are not a believer, their soul goes to a holding place of torment but not yet hell. Hell comes after the resurrection of the dead and judgment. Then those not in Christ will be cast into the Lake of Fire with Satan and the demons for eternity. No one goes straight to hell. There is also no holding place for believers who have no done penance for their sins (i.e. Purgatory). If you are in Christ, your sins have already been forgiven. As Paul wrote, "to be absent from the body, is to be at home with the Lord (i.e. in heaven)" Penance, is not required in Scripture.

So the Catholic church hopes and is encouraged that all unbaptized children will end up in heaven but still entertains the hypothetical possibility that they will be denied the "beatific vision" and linger in a place known as limbo. This quandary exists because the Catholic church believes baptism is necessary for salvation.

 

[Valletta]

This quandary exists because the Catholic church believes baptism is necessary for salvation.

Catechism of the Catholic Church:
1261 As regards children who have died without Baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus' tenderness toward children which caused him to say: "Let the children come to me, do not hinder them,"63 allow us to hope that there is a way of salvation for children who have died without Baptism. All the more urgent is the Church's call not to prevent little children coming to Christ through the gift of holy Baptism.
Catechism of the Catholic Church - IntraText

I have highlighted the portion of the Catechism of the Catholic Church that you seem to either be missing or don't understand. If you want to accurately represent Catholic teaching then please quote the Catechism.