What makes everything simpler is registering your children with both of you as parents from birth. In Spain it works like this: when the baby is born, you both sign the acknowledgment of paternity/maternity at the civil registry and that's it. The kid has the same rights as if you were married - inheritance, social security, automatic shared custody if you split up, everything. You don't need to be married for it to be legal.
What you should definitely do beforehand is a living will or testament where you make it clear who looks after the kids if something happens to one of you. It's even more important as an unmarried couple because if there's no paperwork, the law can bring family members into it and it becomes a mess. Once you have the kids registered in both your names, in practice you're just as protected as a married couple.
Plenty of couples do it this way because they just don't feel like getting married, and it works without any problems. The thing most people don't plan for is getting clear on pensions or work insurance - some agreements give special treatment to spouses, so it's worth asking at your jobs whether as an unmarried couple you can name beneficiaries or if there are options for extra coverage. But for having legal kids and guaranteed rights, no complications.