How to make two dogs get along?

I'm babysitting these two kids, and the family has a dog, Dexter.I met these people walking my dog, Jake, the two played and got along really well. The parents asked me to bring Jake over while I was babysitting so he could play with Dexter. The got along well for awhile, but I've been here for hours now and Jake is getting really snippy with Dexter. Jake is 13 months and Dexter is 9 months. Howw can I get Jake to calm down and stop snarling and trying to bite dexter? Right now I have them seperated, but if someone could help soon thatd be great.