Definitely a kid specific one, an adult bag has too much volume and the child will be cold. Plus you (or the child) will be carrying extra weight.
Over here in Europe the best performance/price option I have found is Sir Joseph Kiki Down which come in two sizes http://www.sirjoseph.cz/en/products/sleeping-bags/i116-kiki-down-125-145-773.htm
They also have a synthetic one.
Ps. If it adds something I’m a father of 5 🙂