err, she cannot control the weather...
had she took them the park and the brought them home muddy and soaked you would have gone mad about that...
and 15 mins waiting? hardly a long time... its a good lesson for them anyway really - to take a positive from it - lots of life involves waiting so getting them used it is no bad thing...
next time just keep texting and reiterate times etc - as i say she may see times given as a general time, rather than dead on...not realising you expect those times to be adhered to... i think would see them as an general time - 6.30 is teatime-ish etc ...as kids are finished when theyre finished, and on a day out i wouldnt want to, or expect others to, cut short the fun dead on the stroke of a set time...