What if we would not be able to agree on the "routine decisions" after the agreement has been executed?

You would simply add a provision for a "deadlock break," either by saying "the parties agree to submit any disagreement to mediation" or by submitting the decision to a disinterested third party, such as a therapist or counselor. By doing this you are setting forth the ground rules to follow in case you and your spouse cannot agree. You must also keep in mind that if you cannot agree on the routine decisions, the child(ren) will become very angry and overwhelmed with frustration. When a child hears for the tenth time that "your father and I have not decided yet", they may refrain from asking in the future.