Calis is actually just an extension of Fethiye so only around 5 miles away. There is a water taxi your kids will love only costs 5 lira and takes about 35 minutes. It's a great trip. Alternatively, catch the dolmus takes about 10 minutes. Calis gives you the best of both worlds, a beach resort with a working town just 10 minutes away.
Book a 12 island boat trip from lots of agencies in Calis, you can also hire bicycles. There is a bike club here on CBF where you can pay 10 Llira for a day out with the club.
Take the kids to the Aqua Park for the day, they have happy hours for icre cream etc., and also have children's entertainers.
Tell your children that the latest James Bond Movie "Skyfall" was filmed on Calis Beach. Daniel Craig stayed in the Yacht Boutique Hotel in Fethiye and you can go through for a lunch or dinner.
There is also a water sports centre at Koca Calis Beach where you can try kite surfing, wind surfing etc. or just watch.
if you are lucky you may see the dolphins in the bay and perhaps the turtles.
There are a couple of places in Calis for Turkish Baths etc., All in all Calis is a great place to stay and a wonderful place to take trips from. I think you kids would like Saklikent Gorge for a day out with a meal on the way at Yaka Park where they can see the trout in the pools and eat really fresh bbq. Also if they you stay in the freezing cold mountain water for 15 minutes you can get a free meal.
There is an official change office in Calis, but if you go to Zumrut Jewellery Shop, he always gives a better rate that the Change Office. Also if you go into Fethiye you can use any of the Banks to Change money and they will often give a higher rate, take your passport with you.