Start with a cake mix in a flavor your child will like and follow the instructions for making a sheet cake. When your cake is done and cooled, remove from pan and set on to a tray or a piece of cardboard that is covered with tinfoil. I love canned frosting as I can never get mine as thick or creamy as store bought. Head out to the dollar store and find some of the three to four inch Barney figurines, you will need four. Also look around and check it out for larger Barney figurines or even for the helium balloons (mylar). Even a birthday plate with Barney on it can be cut out to put in the center of the cake. Take your small 3-4 inch Barneys and attach them with frosting to the corners of your cake. Now for the center of your cake, you have many options when it comes to decorations. You can use four more of the small Barney statues to make them look like they are dancing on the cake or you can cut out a Barney from the shiny mylar balloons. If you do this, make sure you cut through two layers of the balloon to give it a little more weight. Lay it carefully on the cake and gently press to stick. Same idea with the paper plate. You can decorate around the edges of your cut out with frosting. Just put some of it with a little food coloring in a plastic baggy. Mix in the bag with your fingers until same color throughout and then cut the corner of the bag and pipe it out onto your cake. Same with the "Happy Birthday", it can be piped the same way.
If you have a talent for working with play dough or modeling clay, you can use fondant, a moldable frosting, to shape Barney and friends figurines for the top of a cake. The Barney figurine might be standing or seated, depending on how you wish to place him on the cake. The rest of the cake can be decorated with simple frosting and a birthday message or a more elaborate scene such as Barney’s clubhouse or playground. After allowing a day for the fondant to dry and harden, place the figurine on top of the cake. One relatively simple idea with a big impact is a fondant cake that looks as though Barney is bursting out to surprise guests. Cover the cake with fondant and cut out triangle flaps that you can bend back to mimic torn paper flaps.