If you are looking for something they can all sit at the table and do, I'd suggest playdough. But here's the rub--I've worked as a director for mixed age group childcare on several occasions, and the truth is, this age group DOES need constant parental supervision. 2 and 1/2 to 6 is not an age that can self-govern well, and with all the activity, you'll need a designated person in charge of the group. My first suggestion would be to hire a couple older kids (think 10-12 years old) to play with the younger ones and sit at their table. OR hire a professional to bring activities, etc. Kids can also be invited to join in the dancing or some other organized games, and they will likely need some room to move.
Last year I was directing a group (this was childcare that a religious group provided to free up the parents to observe the holidays) of mixed-age children. The crayons/paper was the last thing that kept their attention. Kids need to be able to move around, and having a few activity spaces helped. While we had 3 other adults present (and working directly with the children), our teen volunteers were what really made the program shine. Younger children are more likely to want to impress an older kid (think about your 4-6 year olds) than they are to want to follow the lead of an adult. Please encourage your friend to consider this approach ( a supervised one)...it will be worth the money in the long run.