There's loads that would probably interest your eldest DS around Lyme Regis too!
If he likes railways, he'll love Pecorama. www.pecorama.co.uk The mini steam railway is about a 17 minute ride. There's an expansive interactive model railway exhibition, outdoor play area with pirate ship etc... and a train-themed indoor soft play centre. There's a separate indoor play area for children aged 1-3. There are also some stunning gardens which are great for sitting in and enjoying the fabulous view. Smaller people in particular will enjoy finding the Secret Garden and exploring the Sun, Moon and Rainbow Gardens and the Shell Grotto.
We also like Axe Valley Wildlife Park. There's loads to do including hand feeding the free-range birds, handling a snake or a tarantula or meeting the armadillo, zebra, lemur or flamingos to name just a few! There are various play areas all over the park
Nearby, Seaton Tramway is a good day out. They operate narrow gauge heritage trams alongside the river Axe estuary through 3 miles of countryside and 2 nature reserves. The tram journey takes about half an hour, and they operate open toppers, or enclosed saloons, depending on weather conditions. Look out for seasonal events throughout the year, including The Dinosaur Train, from the creator of the Muppets! Pushchairs can be accommodated on most of the trams.
Lyme Regis Aquarium is a small family run aquarium which is very interactive. One of the most popular activities is hand feeding the very tame mullet. Kids lower their clenched hand containing food into the giant tanks and the fish gently suck and prod at their fingers and thumbs. You can also hold a live starfish, touch a crab and a giant lobster.
It's also worth looking at the website for Beer Quarry caves. There's no lower age limit and you can take pushchairs into the caves. Some children find it amazing but some get a bit scared.