L.,
If you have any fear at all that your ex would "run" with your kids, you need to get a lawyer fast. A lawyer could get a restraining order, or get it set up so that your boyfriend has court-supervised visitation only - to avoid the risk of a kidnapping.
I assume from your letter that your ex-boyfriend is the baby-daddy. If he's not, then he has no rights. If he is, then he has the same rights as any father. You need to have this worked out between you, with or without lawyers.
If he comes to his senses, and will sit down and talk about parenting schedules, where the kids will live, who will have them how much of the time, etc, then you might be able to find a mediator who can help the two of you resolve this more amicably, without a lawyer. Mediators are cheaper and have the goal of helping people to find a mutually agreeable solution they can both live with.
But again - if you have any fear at all that he would run with the kids (and if he's threatening it, that's enough for me!), you need a real lawyer, right now. Beg, steal or borrow the money for one - WAIT - don't steal. If you don't know a lawyer, ask friends who have been through a divorce if they were happy with their lawyer, or call the state or county bar association and tell them you need help finding a good lawyer who specializes in child custody issues. If you are impoverished and can't borrow the money for the lawyer, tell the bar you are afraid of kidnapping, and you need to know if there are pro bono lawyers who might help you. There may or may not be, but it's more likely if there is a possibility of violence or kidnapping.
Good luck and let us know how it goes.