Before discussing what it is that makes a craft easy I would like to explain what is that is so often done with crafts projects that are not easy for the children. It may look easy as all they have to do is copy down the exact direction their teacher may be giving for their 4th of July arts and crafts, but when a craft projects is not done with the developmental needs of the children in mind than it is not easy. The way crafts projects are usually done is that the teacher or parent looks through various websites to get ideas for a particular holiday or season. She gathers the materials and tries to get the children to copy the model she has prepared for them.
If you really want to do easy toddler crafts then the way to do it is as follows. First you figure out where the child or children you are working with is holding developmentally. Can they hold a crayon, what kinds of marks are they making with these crayons, how comfortable are they with scissors. Once you know all of these answers then you will now that most of the so called easy craft projects you find online are not really appropriate for most kids and toddlers. The best way to have children love crafts and do things that are truly easy is to give them crafts that they are capable of doing on their own.
In the case of toddlers, this may narrow down your options of what you can do with them Since I am a teacher of teachers of preschoolers, I always tell them that teaching the younger ages are the hardest. Even though they may be the cutest, they are capable of the least. If you are someone who needs their children to come out with perfect representational craft projects then you must learn to accept the children for the age they are and what they can manage with real easy crafts projects.