LHR T1 *A lounge has a children's room (TV, low tables)

LHR T3 SAS / London Lounge has a children's room upstairs (bean bags, I think)

CHC Internation terminal Koru Club has a small children's area off to the side of the lounge (has a high-chair!)

SIN: Checked out a few of the SQ lounges but none had children's areas as far as I know; but there are quite a few areas outside the lounge and there are always fish to watch!

VIE: Nothing in the *A lounge (airport is relatively small)

Generally, I've found that once children are old enough to know what a plane is, and can walk, that there's more entertainment *outside* the lounges than in it. In SIN we found a dead-end area with a large expanse of carpet where our son ran around and danced for about 30 minutes

The benefit of a lounge of course being that they can't escape ...