With a very small budget
If you do not want to spend any (or very little) money on it, I recommend filling a small garbage bag with wet sand and then placing a cushion or pillow over it. I'm guessing if you want to add a realistic touch, you could place some small sticks of wood in the garbage bag to act as ribs.
Unfortunately, the above answer is all based on my guessing; but seems like it would be fun to experiment with. However, the below answer and link will give you instructions to make a DIY manikin that a CPR instructor has made and uses.
Slightly bigger budget
With some patience, time and approximately $14 USD, you can make a manikin out of a plunger, lid, foam, plaster of Paris and a few other common items. I will not go into detail about it, but here is a link to the blog where you can find detailed instructions, pictures and a list of materials.
You mention having the children practice some skills on a human. As long as they are old enough to understand the concept of not actually doing these skills on a person who doesn't need them, I encourage you to teach them on a human. Teach them the anatomical landmarks, how to check for breathing and a pulse and how to open an airway(1) on a real person.
Hope this is helpful to you. If you have any additional questions, please let me know!
(1) Note: If you plan to teach them how to open an airway, be sure to only do the head-tilt chin-lift. If you wish to teach the jaw thrust, that is best done on a doll.