I live in Jersey. The islands are delightful and definitely worth the trip. You could do Jersey and Guernsey over 2-3 nights quite easily - Sark would probably be more of a challenge to fit in too, although not impossible. The only way from Paris is to take the fast train (TGV) to Saint-Malo (3 hours) and then the Condor ferry (1 hour). You won’t be able to bring a French hire car across on the ferry, and car hire in the islands is easy and cheap.

The weather in April should be mild and pleasant, although be prepare for showers! The islands have lovely scenery, great restaurants and cafes and plenty of places to stay. PM me if I can help with any recommendations!