Where are you travelling from? I'd be inclined to say just plod on and keep going til you reach Ayrshire (sorry Dumfries) there's not really much in Gretna.
There are a few nice campsites around culzean caste. The castle and grounds are worth a visit. You could spend two days there. The castle is impressive, great beach, loads of walks, animals and the adventure play ground is amazing. Close by there is Heads of Ayr farm park which is always a hit. The Robert Burns museum was a suprise hit for my two also. The poems went over their head but they liked the walk (museum is set throughout the village which is really pretty) and the gardens and sculptures.
Arran is worth a thought. It's a popular holiday destination for Scots. Getting on a ferry is always fun and the island is beautiful. Auchrannie resort has a nice pool , restaurant with softplay room and a separate softplay facility. Lots to do. You get the ferry from Ardrossan which is just up the coast from Ayr. There is another ferry at the top of the island that connects the island with Argyll which would take you to Oban. You can also get a ferry from Ardrossan to Cambletown and the Kintyre peninsula. Lots of nice campsite there and cracking beaches at carradale and macrihanish.
Aviemore does have a lot going on and you would not be short if things to do but it is a bit of a jaunt. I don't think you could do Aviemore and Mull. Rothiemurchus estate at aviemore has forest camping. There is the Landmark adventure park (use tesco vouchers to buy tickets), steam railway, great pool at mcdonald hotel for public, softplay at the HIlton, nice lochside beaches.
If you go to mull you must visit Iona. I think by the time you get to mull you'll be fed up with travelling and might just want to stay there for a few days.
Comrie Croft in Perthshires is another very popular place so book in advance. There's loads to do in Perthshires and Stirling area. Blair Drummond safari park, Stirling Castle and jail, Stanley farm park, kelpies/falkirk wheel. Glasgow and Edinburgh are an hour's drive from Stirling. You also have easy access to the trossachs which are beautiful. There's a nice visitor's centre at the lodge in Aberfoyle. It has a nature trail with lots to do for the kids. Nearby there is also Doon Hill which it's rumoured where fairies live. There are fairy houses carved into the trees along the walk and at the top of the hill here is a fairy tree where people tie ribbons and make wishes.
Good luck and I hope the weather is good for you x