sealife is nice and not very big! My dc loved the rock pool where they were allowed to hold crabs!
There is a swimming pool in the town if it rains too much!
The Beach is lovely take a football or a cricket set and others will soon join in
Monkey World is ok if you like Monkeys after you've seen one or two its a bit of a trek walking around. (when we went it was hot and all the monkeys were laying in the shade and doing nothing the keepers went around with food to try to make them more active but it wasnt very exciting tbh) They do have a good adventure playground.
you can go on boat rides from the harbour but even if you dont fancy that its nice to walk around the harbour area.
The Nothe Fort is worth a visit.
There is a dinosaur museum in Dorchester!
Timewalk and Brewers Quay
Also a car ride or bus trip away is the lighthouse at portland. Check the height of your LO's though as they have to be a certain height to climb up it
science discovery centre looks good
We visit Weymouth often but never visit everything. As long as its not raining the kids just want to be on the beach!
The tourist infomation centre is right on the fround (near the toilets and the ice creams
HTH)