Kids are tough. Especially that age group.
I suggest that you find your spot for the larger of the groups, if you're photographing all of the children together, with parents, try to have parents or someone hold the child/ren on a lap to keep them in the photo spot whileyou take the shot. This way they will be less inclined to turn their bodies around completely.
Set up the adults or older children into the frame first (unless they are holding a small child) save spaces for them and put them in last.
In my experience - which is on a great scale with dealing with children - not so grand with photographing - as I'm just starting in the business - you have to get the kids within the first several shots as they lose focus very quickly.
You may suggest - after your first few shots if there is too much uncooperative behavior that if everyone gives you a few more smiles, you'll take a FUN picture and ask them to all make a funny face.
That usuallyworks well enough to get a few more shots in.
As you get your first few arrivals to your spot, you may want to take a test shot or 2 to make sure that your're getting good light and determine whether you need to use fill light via your flash, and to be sure of your exposure settings. You want to do this with an actual person in the frame and not just the background scenery.
With the shots of the kids by the river, if you have a zoom lens try using that and getting some candid shots. The subject doesn't have to be looking at you and smiling, sometimes the best shots are the ones where the focus is elsewhere.
As far as money is concerned - you should take omething for your time.
If you are not comfortable asking for a set fee, tell them that anything they's like to pay is fine. I (being very new to this business and very unsure of my skill) usually charge $50.00 an hour. I am quickly realizing thought that that hour turns into a few when you calculate your time in the post process and travel.
I also have found that although I take tons of shots when dealing with kids and groups, the best are usually within the first few shots in each setup.
As I said I'm not a big pro, but my familiarity with children and people in general helps me to be pretty good at getting shots that everyone is happy with.
You're best bet is to go in there confident - at least pretend that you know exactly what you're doing - and have fun. I'm sure ou'll do great. You asked all the right questions so you must have some idea of how to do it
