A parenting plan written in the month of a separation is written by two people who are not at their best. That is not an argument against writing one.
What it is an argument for is building a review into it. Children of four and children of eleven need different weeks, and a plan that cannot be changed without a hearing will be changed by a hearing.
Settle the handover arrangements, the holidays and the way decisions get made. Leave the rest open, and agree when you will look at it again.