Hi! The best kid friendly museums are in Glasgow, although the Museum of Scotland in Edinburgh should keep them entertained for a few hours.
For day hikes, check out the Forestry Commission website (forestry.gov.uk) as they have great waymarked trails all over Scotland.
For castles look at the Historic Scotland and Visit Scotland websites. There are hundreds to choose from although Edinburgh and Stirling are the biggest and best known.
As far as an itinerary goes it might be helpful to know if you would prefer to see all the famous areas (Loch Ness, Skye etc) or if you fancy exploring some of the less well known places.