Help with Spanish play date

So I have finally found someone to come over to read and play with the children in one of our target languages in Spanish. I know a few people here do this with their children. I was wondering how to work it out. I have over thirty children’s books in Spanish. The individual is a recent high school graduate who can speak and read in Spanish.

How would you guide these “play dates”? Would you give the individual specific words/topics and toys to use when playing with the children? Or would you just let it all happen naturally? The only available time for her to come over is right when I am making dinner so I plan on being busy doing this while she is with the children. Is it okay to tell her to only speak in Spanish with the children and to guide the children to respond in Spanish? My children are very familiar with the language but they have yet to get an opportunity to actually converse in Spanish. They have watched consistently an hour of Spanish shows and stories a day through the Internet and Little Pim for over a year. I also practice basic everyday sentences and responses in Spanish as well as very basic vocabulary building through play toys. They often act out the “Salsa” show during play time all in Spanish but its just usually one or two word sentences.

Any suggestions would be great! I am so excited! Thanks a lot!