FQTV is correct - for ff-members based in NorthAmerica
Frequent-Flier-programs for members residing outside NorthAmerica often other rules for lounge-access apply (example: UA Premex/1K in Europe receive a complimentary RCC-membership).
As your children will mainly need lounge access 2 to 4 times per year only, probably in LHR, the Diners club card may really be the card of your choice as they offer lounge-access free of charge in LHR.
My Swiss VISA (Gold) credit card (bank: UBS Switzerland) provides me and one partner (Gisela) with a free Priority Pass membership (as long as I keep that VISA card) plus 4 free lounge-entries per calendar year (additional entries cost me US$25/entry) - May be a credit-card in your country of domicil has some similar offers? (And as one of my daughters second name is also (Maria-)Gisela she had no problems of using that 'Gisela'-Priority Pass card on their recent rtw-trip ...).
[This message has been edited by Rudi (edited 07-15-2000).]