Moorhen chicks - advice?

[Anyanka]

Hi, my daughter rescued two moorhen chicks from a greyhound paddock (and therefore certain death) yesterday. They are now in an old vivarium with a heat mat, being fed on hard-boiled egg and well-soaked greyhound food. We also offered them brandling worms, but those were rejected.
I found a good internet information sheet about hand-rearing them, but have a couple of specific questions:
How can I figure out how old they are? I don't have scales capable of weighing something so small and light.
When do they need to have access to a pond? I can't put them out on mine because of our dogs. One of the little chicks sat down in the tiny waterbowl in the vivarium and snuggled down as if that was where she needed to be...

Any advice much appreciated!

[Mungo]

Awwww that's great that you managed to save them.
I don't have any advice but hopefully someone will point you in the right direction soon.

[Anyanka]

Thanks, Mungo! They're doing very well, looking livelier today than when we found them. My younger daughter Rhianna (17) has spent hours feeding & watering them. I bought a 10kg bag of chick crumbs which should be more than enough for two tinies... Rhianna dips her finger in water, then in the crumbs and the birdies love it.

[FUDGEY]

A moorhen egg is slightly smaller than a golf ball. If the chicks are about this size they will probably be under a week old. Good luck

[Anyanka]

Thanks, Fudgey! In that case they are older than a week. I took some photographs of them yesterday, although not with a golfball for comparison... This one's the best shot:

[FUDGEY]

they are well cute. They won't be much older than a week going by that photo. You are doing a great job.