This pedal screws in the opposite way and will be a very tight fit, it will be easier to insert laying the bike on its side.
For these sizes and the age of the child riding these sizes, they are likely to lack co-ordination. As a safety measure, the 10” and 12” bike has an automatically breaking back wheel when the child stops pedalling. This means If they are going down a hill and/or are unable to apply the front break, if they were to stop pedalling the rear wheel would begin to brake and slow them down.
There are two different size of clamps, the larger one is for the seat and the slightly smaller one is for the handlebars. (you may need to loosen both of these slightly to fit over the bike frame)