With my original visitation agreement, the judge said that if it is ordered, (always bring that order with you in case...) that there are specific p/u and drop/off times then to be fair is to give the other parent 15 min. then he/she forfeits that particular visitation. Hold true, do not back down or you'll just be setting up for a lifetime of inconsistencies. And right now the kids need as much stability as possible.
Good luck with everything