G,Day Jon,
Not sure how old your kids are what their interests may be but:
Good Wheather:
1. Ball - soccer, footy whatever they are in to.
2. cricket bat and tennis ball (and a couple spare for when they lose the first one in 5 minutes)
3. totem tennis - (stops the ball from annoying the neighbours)
4. boogy boards
and you are already chucking the bikes in, so that lot shouldn't be too bulky and should keep them going for a few days.
Bad Wheather
1. cards
2. board games
3. sketch pad and pencils
4. lego or toy cars
Then tell them thay can take one other toy each of their own choosing as long as its practical.
Besides that,
fishing line and couple of sticks makes a bow and arrows
stack your beer cans up and they can play skittles with the tennis ball
or take a magnifying glass or lend them your glasses so they can burn ants
etc etc
Shouldn't be too much of a drama for just 4 or 5 days as long as it isn't raining the whole time. #Wouldn't take too much, whatever you decide on; they'll probably spend the first 3 days exploring.
Cheers
Jeff
PS don't forget a camp chair for everyone.