Scotland does have so much on offer and it is always hard to pick what are the best bits to visit.
The south of Scotland is a very beautiful area with so much history. There are Castles, towers and Abbeys to visit. In the town of Melrose there is an Abbey, where Robert the Bruce's heart is buried. They have some lovely gardens and a nice riverside walk. There is also a Roman museum in the town as not far from Melrose is the site of the Roman Fort Trimontium, which means 3 Mountains as it sits at the bottom of the Eildon Hills. The Eildons are 3 hills which you can walk up, but aren't too high so children can easily walk them. The views from the top are amazing and you can take in the beautiful Border countryside.
Abbotsford House sits near Melrose and was the home of Sir Walter Scott. Not far you have Scotts View which is a stunning view of the Eildon Hills and where Scott used to sit and get inspiration for his novels. Further down the road you have Dryburgh Abbey where he is buried.
In Eyemouth, which is on the coast heading down towards the English Border there is Gunsgreen House, a smugglers palace and a great place for all the family and one that kids love.
Further up from Eyemouth you have St Abbs Head Nature Reserve with its breath taking coastal headland with dramatic cliffs which is famous for its seabird colonies.
Hope that helps with just a little taster of what the south has to offer.