Unfortunately you are not allowed to have a picnic inside the Eurodisney park
, but there is a dedicated area for picnics right outside the gates.Make sandwiches or buy some from the local shop and leave them in the car. Freeze bottles of water beforehand and store them with your sandwiches. The icy water will keep your sandwiches cool, and the will ice have melted into nice cold drink by the time you are ready to have your picnic.