Like weather throughout western Europe, Paris in April can be extremely variable. When visiting DLP at Easter time I've had snow on one occasion, but on another it was shorts and T-shirts during the day. This makes deciding what clothes to pack quite tricky! As Richard said, DLP has lots of areas around the park which can shelter you from the rain, including a covered walkway more or less from the front of the park to the back. If you do find yourself in the parks during the rain the one consolation is that crowds will be smaller.
If you are still deciding what time of year to visit DLP I would recommend you think about the summertime, between approx 10 July and 1 September. This is when DLP has its extended summer opening hours. Although crowds are higher at this time of year, the long opening hours does give you plenty of time to avoid the worst of the attraction queues. Disneyland after dark, with fireworks and a new parade (Fantillusion) every night, is a wonderful experience.
Regards
Rob