Nothing! It can't be done!!
3.5 year old girls don't belong on a boat, so I guess that you will have to sell her (the boat, not the daughter
- although I can think of one member that might make an offer for your daughter
You are in luck! I just happen to know someone looking for a boat.
Seriously, I agree with JimHawkins. She needs a small area of responsibility on board. Give her a job, and lavish praise on her for doing it well.
Here is one idea: have her plan what you will do when you go ashore (Jewell / Damariscove/Eagle Islands). Help her to understand that half the fun of sailing is planning what you are going to do.
Put her in charge of watching out for motorboats, Rocna Anchors, Ketches and Sloops. Give her a notebook, and have her keep a running tally.