I did family shots of kids exactly this age a couple of weeks ago. In retrospect, I really wish I'd thought it through more beforehand. It was a freebie as it was an assignment for a photography course, but I was still disappointed I didn't get better shots to give the family. Lessons learnt were:
- Scout out the venue beforehand and choose a good background. Get some posed shots here while they are being co-operative. This way you always have at least one decent portrait. Read some books on portraiture to get positioning right - and remember to check there are no legs facing towards you, no hands taking over the shot etc. I panicked and didn't direct them at all - so grateful was I that they were actually sitting still for 5 seconds!
- Beg/borrow/steal a longer lens so that you can get lots of grab shots from a distance. Some of my grab shots were better than the official portrait (because I didn't pose it properly, whereas grab shots don't need to be posed).
- I would normally do shots in the family's home/garden but for children this age I think you're better at a park - they can't run off to their room or other parts of the house, and if you take them somewhere with swings etc there are good photo opportunities with the bright colours.
- speak to mum and dad first about what to wear. Make sure outfits aren't clashing.
- Speak to the kids, build a rapport with them as best you can - they respond so much better if they're having fun. Make them jump around, play games, tickle each other - anything to get them laughing and joining in as much as possible.
Good luck!