Been there quiet a few times and very similar to haven although.not as full on with the entertainment,They changed ownership a few years back(I believe Park Dean own it now) but no big difference.
Beach is not up to much bit the surrounding areas are beautiful.Plenty to do for the kids in neighbouring towns.
Farm Park nearby..you pass this on the way into Southerness
Kirkudbright Zoo (spellng??)
Dumfries has some smashing shops
And an Ice Cream place come adventure playground at Dalbeattie.
Make sure you pick=up some pamphlets at the Dumfries tourist office.
And the most beautiful "HILL" in Scotland..Crifel (I was told) is Scotlands largest hill...only a few meters too short to be classed as a Mountain (Don't know how tru this is but it sounds good )

Couple of niggles...you have to book for the swimming,get to the club early if you want seats (same as everywhere) and the local chippie is pretty rank.

Strange weather up there..sometimes when you think its going to chuck it down the clouds seem to roll over and lash down further inland..

My kids are grown up now but we still have a bank holiday up there every year...it knocks 10 years off me every time. Aaaggaaaaa Doo doo doo !

I was born & brought up not far from Southerness, and know it well, or "knew" might be closer, as I haven't been for a few years. If you're a golfer, the local course is OK, but there's a course a few miles away at Colvend that can be challenging. Lots of little villages along the coast if you want to get out & about - Colvend http://www.colvendcoast.com/ Rockliffe & Kippford (linked by a clifftop path), New Abbey sits at the foot of Criffel, and has Sweetheart Abbey, and the Corn Mill to visit
http://www.historic-scotland.gov.uk/properties_sites_detail.htm?propertyID=PL_220
http://www.dumfriesmuseum.demon.co.uk/sweet.html
http://www.historic-scotland.gov.uk/properties_sites_detail.htm?propertyID=PL_283
Also just up the coast is Sandyhills Bay, which has a much better beach than Southerness, so you might want to think about that.

Great, thanks for the info, I've got a 5 yr old to try and entertain so looks like there should be plenty to interest him, it a shame about the booking for swimming, not sure how that will fit in with our plans if the weather is unpredictable but Im sure we will work around it