What strategy should FLL teams use to pick their next robot mission?
First off, that website is horrible, so much clicking on expandable things, a terrible UI.
The PDF download of the rules is better.

Looking through the missions I notice a few things:


None of the missions have significantly bigger point values to make them obvious choices over other missions.
The missions are all based on end state and cannot be repeated for multiple points


So it appears you should simply try to maximize the number of successful missions accomplished in the limited time period.

Some missions should obviously be combined since they are handling similar pieces and would be easily doable with the same robotic manipulators.

Such as:


Moving pipe sections for: pipe removal, pipe install, pipe construction
Moving Big Water pieces for: fountain, flower, and water collection
Manipulation of levers or handles: flow, water treatment, faucet
Picking up, moving, and aligning pieces in the field: pump addition, rain, manhole covers, sludge, water well
Moving objects from the base onto the field: tripod, water well, slingshot


Also some missions must be done before other missions:


Manhole covers removed before a tripod is placed
Pipe removal before pipe install


The best bet is to pick a group of tasks that a single robot build is likely to be able to do well and go from there.

Some thought might be given to completing the furthest missions first, so that traveling past already completed missions doesn't mess something up.

You might start by sorting the task into groups like I have done, try to judge the inherent difficulty of the mission, and rank them comparing with point totals.

On a final note: As this is a competition for children to learn about engineering and robotics, you should really just explain the rules/different objectives/point values and help the kids pick a strategy themselves.  The planning process itself is as much engineering as the programming and building of the robot.