Last time that I've visited Paris I was able to go for Eurodisney and I saw some people with natural sandwiches on their bags, is it allowed to enter on Disneyland parks with some food? It's very expensive to buy inside the parks.
I have found this post about it. It is in French but I'll translate the general idea behind it : http://www.disneylandparis.fr/faq/restaurants/alimentation-boissons-exterieur/
So it isn't allowed to bring with you large bags full of food to organize a group picnic inside the park. They don't explicitly say that it is allowed to bring some water or sandwiches but as it isn't explicitly forbidden, then you can safely assume that this is implicitly allowed.
The general idea is that they want to avoid group of 10 people sitting around and starting a picnic in the middle of an area. But drinking your own water or even eating a sandwich is ok as long as you don't start using large spaces. Still, don't expect to have some free tables or seats inside the park to ease this...