Change separation agreement without court involvement in MA?

In my opinion, it's always better to go through the court when attempting to modify a separation agreement.  Any agreement you make on your own without the court's involvement is not enforceable or binding.  That means that, at some point, your wife could change her mind about the change in visitation and refuse to allow you to see the kids on the new schedule.  That could be disruptive for your kids and harmful to your relationship with them.

This issue becomes more important when divorced couples contemplate altering their separation agreement for child support payments without involving the court.  That is definitely a no no.  If one spouse reduces child support payments without asking the court to amend the order then, in theory, the other party could ask the court to hold that spouse in contempt for failure to make court ordered child support payments.  Either on your own, or with the help of an attorney, draft a complaint for modification and file it at the appropriate court.  Given you are in agreement on the issue with your Ex, this should be fairly easy to do and will cover your backside if things between you and Ex turn sour at some point.  Hope that helps.