Most if the time my answer is going to be “get it in writing.” While you may have the most amicable divorce in the world, you never know what the future may hold. Contracts fill the gaps left when human trust fails. If this is a one-time, or two-time situation, a verbal agreement should suffice, but if this is a temporary change of duration (summer school vacation) you should have a confirmatory writing. Otherwise, you may find yourself agreeing to an ongoing arrangement without intending to. Lastly, if this a permanent arrangement, I highly recommend that you change the parenting plan in writing and file it with the court so that it is made an Order and is therefore enforceable.
Whether you are working out an initial custody agreement or need to modify one that is no longer working for your family, you are welcome to discuss your individual situation with one of our experienced family law attorneys.