If you have children together married or not, the issues and the resolution of issues are identical. Married or not, if you are fighting over custody, possession and child support, you must file a lawsuit similar to a divorce case. It's like a divorce with no property issues.
"Suits affecting the parent child relationship" (SAPCR's) are custody suits and can be more expensive than a divorce with property issues only. More and more fathers these days are demanding equal possession time with their children, and many fathers are seeking primary possession. The law in Texas says the Court or jury cannot consider a parent's gender as a factor in determining best interests of a child when it comes to determining custody and possession issues.
And if you think a prenup will keep you out of court for child issues, think again. You cannot contract for child issues and if you do, such contracts are not enforceable in any court in Texas which is why we don't put child issues in prenups. So please understand, if you aren't married but you have children together, you must still have a lawsuit to settle the "kid issues" as we call them. This isn't to say you can't make agreements and have an agreed order settling all the issues, of course you can. But if you cannot agree on all the issues involved in these type cases, then you will have a lawsuit on your hands and you need experienced counsel to skillfully guide you through these matters and advocate for your and your children's rights.