Ae green/blue. The new blue DH section(Collins Folly)is smooth enough for the wee ride and not too steep. Most of the blue is forest road with some flat stuff through the trees lining the river bank. Good cafe with showers.
Mabie blue only has 2 singletrack sections which should be ok, the rest being forest roads.
If you can access the local cycle route,
http://www.visitdumfriesandgalloway.co.uk/pdf/cycling/nithsdale-cycling
http://www.dumgal.gov.uk/index.aspx?articleid=2538
http://www.gosmartdumfries.co.uk/index.aspx?articleid=9728
http://cyclingdumfries.wordpress.com/summer-rides/heathhall-explorer/
You can cycle out to Heathhall and access the forest from the cycle track. There is a short surfaced track in the forest along with a few surfaced walking tracks + some lovely rooty stuff we use for kids races.
The forest is adjacent to an old airfield where ther is a museum.
http://www.forestry.gov.uk/newsrele.nsf/WebPressReleases/6D9345A982280EA680257BD000402D2D
Not been round Dalbeattie blue but I am sure it should be ok, loads of forest road to go on anyway . Some of the red nearer the car park end would be ok. You could cycle through the forest out to Colvend then down to Rockcliffe beach and back for a good day out on the walking route(brown posts)
There is also Drumlanrig
Just a thought? Do you mean seat on bike or a wee bike?
If bike the riverside stuff at Ae, maybe carry it up Collins Folly otherwise the route is too steep climbing wise
Mabie blue would be too long but there is a small skills area
The last section of the Dalbeattie red is a favourite with kids over that way. https://www.facebook.com/specializedbicycles/posts/10201544654721661
Heathhall is perfect 😆