What you need to get:
Something to stir the water and powder together (you will throw this away when done)
Then they will need a piece of foil large enough to line their shallow container. Crumple the foil some first before lining your tray/container (it will give an interesting rock like texture around your fossil) Be careful not to rip it. You don't want the plaster to leak out any holes.
Make your lump of clay wide and long enough for your dinosaur to fit. Don't think it out too much, you don't want to hit the bottom of the clay while pressing in your dinosaur. You will want to press the clay into the foil to try to prevent the plaster from seeping under the clay. It will make removing the clay easier later on. Lay out your dinosaurs. Let the kids decide what they want to do. Some of the kids at the party took sticks and pressed them in. Some drew their initials into it. Here you can see I used several types of dinosaurs. Some of the kids did all of the little dinosaurs, "like a mass grave." Others added a little dinosaur next to the large one and said "it's Mama and Baby." Kind of depressing if you ask me but they were having tons of fun letting their stories come to life. In the picture above I picked some of my parsley because I thought it would leave an interesting pattern. Look around and see if you have something that you can imagine is prehistoric looking.
When making your impressions, you are going to have to help your child make sure the whole side of the dinosaur is pressed firmly into the clay. You may have to rock the dinosaur as you press to get the entire profile. The neck, tail and spine at the pelvic area are usually the places that don't just press in. We had to show the kids how to press on the legs and then all along the dinosaur to make sure we got a good impression. If you are careful you can put the dinosaur back in place if you need to go back over an area. Some kids will find that they have rocked their Dinosaur two much and made a double image. The beauty of this project is they can mash it up and start over till they have something they like. This picture is a bit deceiving because it almost looks like a positive image, when it is in fact a negative image. When you pour the plaster over you will get the positive image and it will look like a skeleton that has been partially excavated.
Now the kids have a fossil that they can proudly display in their room on a plate stand or on a wall hanger of some sort. We didn't have the kids paint them up at the party. This project takes up a bit of time as it is. I think they look great just the way they are. Mine will remain unpainted. All of the kids were excited about the fossils they made. This is crafting made cool! Boy and girls alike will love this. I know from personal experience.
My brother was here for my son's Birthday party a few months ago. He did a fantastic job running the fossil making table for me while I was able to focus on making sure other things were ready to go. I will also say that he had as much fun as the kids doing this. He does not often have opportunities like this to make something with kids. Grandparents are also usually happy to help out with projects like this. My Dad also got into this project and helped out. I also noted that many of the parents were standing by to see this project in action.
I always recommend getting other adults involved in manning an activity like this. Give them a quick tutorial on what the project is and have one or more adults work with the kids. You will thank me for delegating some of the party activities. At this party I asked a friend to take pictures. It ensured that I got more than just a handful of party pictures. Isn't that what family and friends are for? They make my life so much easier during parties. Too many years I did all the work and didn't have any time to socialize because of it. Because I delegated this on the last party I was able to socialize far more than any other party we ever had. I never knew parties could be so stress free!!