@Jessca95
Sad, and will pray for your circumstances. Your concerns are not without merit. If he has shown little love for them in the past, unless he goes through some change in the future (Comes to Christ Jesus), he will show little desire to be with them now or in the future. I have a feeling just from the initial post, that the reason he would want the 50/50 custody either has to do with his family (which you don't mention) or child support payments.
You should seek legal advise. Not being married (implied), probably limits the rights he has, but that is something you need to check on. As you acknowledge, the children are you priority. They should have a knowledge of the father, and contact as long as their safety is not an issue that concerns you should they spend time half the time with this ex-boy friend.
First start praying and asking God for forgiveness & guidance in what you should do. Without Jesus in your life, and Him being a priority, all else will have a different outcome. Yes your children are a blessing, a treasure to hold on to, but if they're going to benefit from what they have had no control over, it will come from your own relationship with Christ and the example you now set forward for them to follow. For it appears they will not be receiving it from their earthly father.
Praying that it works out for you.
With the Love of Christ Jesus.
Nick
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