Well it depends on the overall ages. This time round mine are 1.11, 2.1, 2.2, 2.7, 3.3, 3.5, 5.11, 6.7 & 7.4. I'll only have 6 of these at a time and it will be a mix of 4 of the younger ones and 2 of the older ones each day (different mix on different days). I'm looking forward to it being much easier and really, it's my own 2.1 year old I'll have most trouble from!
Last year in the summer hols I had:
13m, 19m, 2.3y, 2.5y most days along with either 4.11y, 5.4y or 5.7y and my 6.4y ds. For this mix the triple buggy had to be used and bags of smaller toys taken out. Harder work with 13m old ds trying to eat bark etc. on picnics but still good fun. Lots of feeding ducks and stuff too.
It really depends on how many of what ages you have.
I find that walkers, even new ones, like to collect treasure (leaves, mud, twigs, pine cones etc) - I give them a nappy bag or they use their sandwich bag from lunch, with their name on.