Personally I would send out a memo now that this is the new 'policy' for dealing with late fees and than when you renew your contract if you want that policy in there for clarity than for sure add it!

My late policy is $1/minute to compensate my over time but it is also followed by the statement. Please note that frequent lateness past your contracted time will result in TERMINATION of this contract and forfeiting of the security deposit!

I do not want people thinking they can just pay 'extra' and use as much time as they want ~ IMO lateness is just NOT ACCEPTABLE ... plan your day to arrive to pick up your child before your contracted time is up! I have made exception for true family emergencies where there had been a unexpected hospitalization, death in the family and the client had to find alternate person to come pick up while they deal with it ... but got held up at the office, the roads were bad due to the weather, there was traffic and so forth specially without a phone call and just left 'waiting' ~ sorry but that will be a $1 per minute and 'strike one' ... frequently try to use that as excuses and you would be done here!