They don't have Airport Express travel passes for children, so that's definitely a horrible deal to buy those for your kids.

For 2 adults and 2 kids for just the roundtrip on the Airport Express, if you buy a "Group of 4", it will cost you HK$500 ($250 each way).

Or if you buy a "Group of 2" tickets for the adults, that'll cost HK$320 ($160 each way), and the kids can buy two roundtrip tickets (good for one month) for HK$180 ($90 per child). So, still the same HK$500. But more complicated.

Those are fares to HK Island. A bit cheaper to Kowloon.