You've received some great advice on your first question. As for the second question, the answer really depends on the way you present your effects and the age group for which you are performing.
For instance, I specifically designed my sponge ball routine for children 6 yrs. of age and under. It's quick, visual and waaay over the top. If I showed that same routine to an 8 year old, he would think I was an idiot.
You see, different effects and presentations will do better with some age groups rather than others. That's because children's attention spans, maturity levels and life experiences change with age. To select your material, and present it to its fullest, you need to understand the psychology of children.
As for the cups and balls, this effect goes extremely well with kids 8 years and older. At that age, the kids won't have any problems standing for a few minutes to view the effect, especially if it is near the end of your show. I find the kicker ending, with some really great final loads get a really good reaction with both kids this age and their parents.
But be forewarned. Kids in this age group are very critical. If you are going to perform the cups and balls, you better have a great routine and perform it perfectly - otherwise, they will eat you alive!
As for the cups and balls, I'm not sure exactly how well it would work for kids and at what age. I think a simple routine, as I recently saw, would work for kids at the age 7 level. But I also saw a complicated one and the kids 5-8 didn't catch on too well. Since they're young, you need to keep it simple so they can see and understand what's happening. However, perhaps somebody who already performs this effect could elaborate further.
As for sponge balls, 5-8 years olds are great to perform this for. Include audience participation if possible. I've performed sponge balls many times for kids at this age and it's never failed to work. Even adults like the effect.