This activity doesn't need any preparation in advance of getting started with the children. A dry and sunny day might be the easiest option, but do consider going ahead with this activity even in the rain - it might be interesting to see if you get different results depending on the weather...
At this point how you proceed with the activity will depend a bit on the ages of the children involved. It's a great example of a child-led activity, so don't worry too much about steering their exploration in any particular direction. Some things you might consider doing include:
16 - 26 months
Mathematics, Numbers; "Says some counting words randomly."
22 - 36 months
Physical Development, Moving & Handling; "Squats with steadiness to rest or play with object on the ground, and rises to feet without using hands"
Mathematics, Numbers; "Recites some number names in sequence"
Understanding the World, The World; "Notices detailed features of objects in their environment"
30 - 50 months
Mathematics, Numbers; "Shows an interest in numerals in the environment."
Understanding the World, The World; "Can talk about some of the things they have observed such as plants, animals, natural and found objects."
40 - 60+ months
Mathematics, Numbers; "Counts objects to 10, and beginning to count beyond 10."