The spice in jalapeños can vary, but almost all of the spice is in the seeds and veins, so you can remove those to limit the heat. If you want a kid-friendly sauce with no spice at all, remove all the seeds and veins. You can even use red bell peppers to make the sauce (zero spice). This is a great way to go if you want to make a portion of the sauce without spice and some with. Add red pepper flakes to taste to the portion of the spice you want to be spicy.
Feel free to halve this recipe or double (or triple!) it. It keeps for 3-4 weeks in the fridge, but never lasts that long at our house.