more than likely the child was playing up more because you were there because add/adhd children thrive on getting attention, so if they cant get attention for being good a lot of children will pick up that bad behaviour will get a reaction so like wi fighter said only reward the good behaviour, small tasks and rewards like a chart and stickers then something after so many stickers is a good idea, if you are only visting monthly just tell them next time you come because a month is a long time to a any child but a month is a very long time for add/adhd children, i learnt the hard way and i am still having problems, read up on adhd/add there are some good sites out there, it will help more if the child recives there meds regularly ritilin will only last for 4hours at a time ,my son changed to concerta its a slow release drug
good luck