It just depends. The "big kids" games like Jurassic Park, boxing, etc. were three and four credits and then the "little kids" games were two. You purchase a refillable playing card. We bought our son a $20.00 card and it lasted through several trips.
You are better off buying the larger card at the beginning because you get a lot more points for the money. I don't remember if it was 20 or 25 but that is the way to go. So 60 or 75 for your kids should do it for several trips to the arcade, unless you plan on spending hours in there.