Will 40km E-Bike trip in Texel Island(Holland) be hard for untrained people?
Short answer: No you'll be fine and (if the weather allows) you'll have a wonderful day.

Longer answer: You are on an e-bike, 40km is about two hours of riding on an e-bike. This is independent of fitness, because you are on an e-bike. Saddlesoreness (is that a word?) might be an issue, but given that you have 8 hours total you can bike short bits with long breaks in between to rest your tender bits. If it turns out range is an issue you'll come across dozens of places where you can use those same six hours you have left to charge your batteries. Texel isn't crowded by dutch standards but it is by no means empty. 

The highest point of Texel is 15.3m above sea level, the lowest point about 1m below, so hills are not an issue either.

More than your fitness level I expect the weather (rain and/or wind) might be an issue.