Castle Rushen is a great castle, there’s rooms to explore, they have knowledgable staff around and they have even kitted out most of the rooms to give you that authentic feel.
Kids will give it mixed reviews, as it is probably quite a scary place for a little person! But if they aren’t bothered by that they will really enjoy having an explore. You can access pretty much the whole castle and there is a lap which takes you from the courtyard up to the top and back down. Keep in mind that due to budgetary constraints, they did not install a lift in the 17th century, so there are a lot of stairs!
You can get up onto the Castle wall and walk around, and have access to the gardens which is handy if you have a picnic with you on a nice day.
There is accessible car parking in a few locations within 30 seconds walk of the front door.
This is a Manx National Heritage Site, so you will need to have a pass/ACE card, or pay the fee to enter.