Extra Money:
Those kid actors on the Lucy Show might or might not have earned extra income from the commercials. In those days, an actor under contract often had to do such work as part of their obligation, with no extra pay. All the ads you see in old magazines, where movie stars pitch Lux Soap and Pond's Cold Cream, etc., were just part of the job. The stars didn't necessarily use the product or, in some instances, even know what they were posing for when the photography was being done. Some stars, like Joan Crawford, saw the practise as good exposure. Others, like Bette Davis, hated it, and complained every chance they got. This system held up through the 1960s, though really big stars could sometimes get out of it by that time. But that wasn't always true - Kim Novak was worked very hard for ads, doing a major campaign for the Edsel's introduction, as well as for a mattress company.
Motion-picture contracts during that time were pretty close to legalised slavery, although the actors were well remunerated, at least those who were stars.