Although children with chronic illness have lower parent-reported general health, their life satisfaction appears comparable with that of peers without chronic illness. With this study, we provide evidence that chronic illnesses do not preclude children from leading happy and satisfying lives.

But regardless of the health of the children, two factors stood out in their happiness: the family income (greater means happier children) and the amount of stress felt by the children (more stress, less happy).

https://psiquiatria.com/trastornos-infantiles-y-de-la-adolescencia/un-estudio-demuestra-que-el-tdah-no-afecta-a-la-felicidad-de-los-ninos/

https://pediatrics.aappublications.org/content/early/2019/05/02/peds.2018-2988