You can blame the SNP for any ferry issues, I certainly do!
If OP was going today, they're probably heading back to the ferry now, so I hope you had a lovely time. Otherwise, here are a few more ideas. I go to Arran frequently to visit my parents and I've known it all my life. I love it.
Big tip I have is to make sure to have an Arran Dairies ice cream. You can get them at Taste of Arran shop in Brodick where you get the tickets for the crazy golf, right next to Arran Active. They also sell Arran cheese, Arran beer, Arran jams, mustard, chutneys etc, Arran Aromatics* products, Arran whisky and Arran Gold, and Wooley's oatcakes, all of which are great.
Wooley's the Baker is further along the front and sells lovely pies, cakes and all sorts of lunch stuff as well as the wonderful oatcakes. Very near it is James's Chocolate Shop which is good (they make the chocolates out the back and you can watch through a viewing window) but expensive. Little Rock, the cafe next to the crazy golf ticket place, is good for coffee, brunch, lunch. There's a takeaway pizza place beyond Wooley's which is usually pretty busy in summer and also sells ice cream.
I think the Whiting Bay suggestion was excellent. The bus service is usually great, but you need to keep an eye on the time to make sure of not missing the return bus to the ferry terminal in Brodick.
With more time, I'd have said go to Kildonan as well, which has a lovely beach, and is also served by the bus. Lochranza is very atmospheric but doesn't have a lot for kids to do, I suppose. (For adults, the distillery tour is interesting.) Standing Stones at Machrie, King's Cave, lovely beach at Sannox - there's a lot to do on Arran if you like walking and beautiful scenery.
I love the walk from Brodick to the castle along Fisherman's Walk. I understand the castle is still open while it's having building work done. Not sure I'd bother with paying to look round it, though. The grounds are lovely and as mentioned above there's a good adventure playground. There are very good walks up through the country park. I particularly like the walk into Glen Rosa but it's quite long. Climbing Goatfell is of course an option, but one that you need to be prepared for - appropriate shoes and clothing, emergency rations, plenty of water, good levels of fitness. For me (fairly good walker, but far from superfit) it took most of the day and I was very, very tired by the time I got back down, but I'm very glad to have managed it (some years ago now).
I've got to mention the Arran Heritage museum on the outskirts of Brodick, which is our standby for wet days. Some very interesting displays. Has a good cafe. Does a lot of special events over the summer months, usually on Sundays, I think.
*It's not called that now, is it? Can't remember the new name.