It's just a case of identifying the hazards and then guestimating both how likely they are and how severe the harm would be, and then combining the liklihood and severity to get the risk. Here's a simple way to do it: Rate the liklihood of the thing happening on a scale of unlikely=1, possible=2, likely=3, and the severity of the harm on a scale of sob=1, wah=2, arrgh!=3. The measurement of the risk is the product of liklihood and severity, and if it's over 5 you do something about it.
Like this:
Risk Assessment for a School Visit
Hazard Liklihood Harm Severity Risk
Kid falls into water butt possible=2 Kid drowns Arrgh!=3 2x3=6
The risk is 6 which is too high and it tells you that there's a possibility of a very severe accident and you need to do something about it. So if you have uncovered water butts on site then you need to have covers made for them. Then assess the risk again until you get the score below 5
Do this for all the hazards you can think of, for example kids eating some poisonous plant, poking their eye out on a garden cane, drinking pesticide, getting a clip round the ear for walking on a grumpus' plot, and of course getting buggered by the local perv. If you can't reduce all of the risks below 5 then the risk to the children is too high and can't go ahead.
It's a useful exercise because it spots genuine problems that common sense appears to miss, and it does prevent accidents. It's also useful to be able to demonstrate that the health and safety of the site has been managed.