Stay in the Holiday Inn right next to it, park up for free, it's got a pool to spend an hour in with the bairns. Bus ride into Edinburgh centre or a few quid in taxi for bait.
This, go now while the kids are young and happy just to go to the pool and ride bikes all day. If you go when they are older it will cost you a fortune for all the extra activities.
Took my son to Edinburgh zoo when he was young and he preferred the hotel room
Don't think you need grand gestures with a toddler and a little un.
The Thomas the tank engine days are canny when they come around
Stop here and spend a few quid in adventure valley with the money saved