The Glentrool (blue?) loop has some pleasant singletrack and is a 30 minute xc adult blast so perfect distance for a keen kid. Think the cafe is still going and there is a new community hub / cafe / accomodation – no idea what it is like. https://www.glentroolhive.co.uk/the-coorie
Mossyard beach is good for clambering around, rockpools and sand at low tide with some accommodation. Various static caravans around that area. Google maps is probably as good as anything for finding accommodation options. We stay at Galloway View in Gatehouse of Fleet – can’t remember if the bed in their little “bothies” can be set up as twins.
As mentioned stops for wild goats and deer on the road to Clatteringshaws. There is a deer landrover safari thing at Sandyhills which is a bit spendy but also friendly cafe which is free to visit and sometimes has views of the deer.
We’ve seen red squirrels and woodpeckers on the feeders at Kirroughtree (hide on the way to old / second playground). In June I fleetingly saw the Pine Marten on Kirri red (“nutcracker” section). Also occasionally seen red squirrels biking at Dalbeattie and Mabie.
Red Kites and some other wildlife hides around Laurieston, Loch Ken and Threave.
Galloway Smokehouse has an American style diner if you want something different for burgers and milkshakes in an evening.