I would be reluctant to photograph a five year old around sunset - if they have been up since six am and been to school they are likely to be tired and un-cooperative. If you can, then first thing works really well as everyone has had a good nights sleep (theoretically) and so should be full of energy. Do make sure they have had breakfast too, nothing like a hungry child for a tricky shoot. You wont have that wonderful evening light but I think that is an easy price to pay for not working with tired children. If you do do something later in the day then make sure everyone has had a proper snack first so you aren't battling against hunger as well as tiredness.
With the ten year old then I would get him involved with something. Find out what he likes to do and then aim the shoot around that. For example, if he is into cricket then have the kids playing cricket together, or get him to teach his five year old sister how to bounce a ball on a cricket bat (you will get great shots if he sees how many keepy-uppies he can do). Perhaps he likes drawing and art so maybe you could get them to do a big picture together. I photographed an eleven year old boy at the weekend and we played pool, poker, cricket, and on the Wii. The variety kept it interesting for him and there were plenty of opportunities for me. Apart from Poker those are all things you could do with a five year old too and some kind of card game - top trumps or snap perhaps - would work well with a younger child.
Usually with siblings there are activites that they will do together or even just get him to play around with his little sister, chase her, tickle her, play hide and seek. If he is involved in something then you are less likely to get the 'too cool' problem. You could go to a playground and have the older boy pushing his sister on the swings and playing on the roundabout together as another idea for shots with the two of them.
In terms of lighting, then natural light is your friend, keep them in the shade if it is a very sunny day.
Good luck and enjoy yourself
Helen