Is the a train from Beauvais airport to Disneyland?
Don't think there is a direct train but what is the best way?
And will need to run on Sunday afternoon
There's no train directly from Beauvais to Paris nor Disneyland. The easiest transfer would be to take the Beauvais Airport shuttle coach bus to Porte Maillot, then taking Paris Metro line 1 to Charles de Gaulle Etoile (Arc de Triomphe) station and transferring to RER A all the way to Marne la Vallée/Disneyland (terminus station).
At Porte Maillot Metro line 1 station you can purchase a single-use RER ticket to Marne la Vallée and it will be good for the Metro line 1 to Charles de Gaulle Etoile station as well. You don't need a separate Metro ticket for this short Metro ride to the RER A station.
This itinerary will operate on Sunday afternoon.