Can a family arbitration clause in a separation agreement be enforced if one party later objects?

Generally, yes, if the clause and the surrounding agreement meet the formalities Ontario's family law framework requires, a party who later has second thoughts about using arbitration can still be held to that earlier commitment. This is exactly why the formalities, being in writing, both parties receiving independent legal advice, and the process being tied to Canadian law, matter so much: an arbitration clause entered into properly, with real understanding on both sides, is intended to be binding, not something either party can simply walk away from later just because they'd now prefer court.

That said, a party objecting later isn't automatically without options; if there's a genuine argument that the clause or the underlying agreement itself is invalid, for example due to missing formalities, a lack of proper independent legal advice at the time, or other grounds that could affect any domestic contract's validity, that's a different question from simply changing one's mind about the process. Because the answer depends heavily on how the original agreement was actually put together, anyone facing this situation should get legal advice on whether a genuine challenge to the clause's validity exists.