This is a risky proposition, on many levels.
1. Legal: depending on where you live, you run the risk of ostracizing your families, your children and your community to outright losing legal custody of your children. Please check the family law in your country or state before even thinking about getting pregnant.
2. Emotional: Adults have feelings and, more importantly, children have feelings. Adults may well decide to part ways and if parenthood is mingled, children will suffer due to separation. Please remember that as adults, we can choose whether or not to have kids but kids don't get to choose what family they're born into. Making a solid commitment to create a loving home for children is a serious business, ethically and, in many places, legally.
3. Financial: if you split up, who will support the kids?
4. Practical: if you split up, where will the kids live? Who will take care of them?
As a single and successfully poly mother of two kids, I'm not just talking out of my a** here. Please do some serious research before you commit to caring for dependent human beings for 18 years or more!