M.C.
I have 4 children...all teenagers now...and admit I had a difficult time leaving my kids with a babysitter. Now I have three daughters who are babysitters. (They are 19,14 and 13) Honestly, whether they are in high school or college doesn't make a difference...it is more their personality and comfort with children. (My 14 year old would be the best babysitter out of my three girls and she is in 8th grade. Honestly, she could handle the three children much better than my 19 year old.)
I would definitely use word of mouth to find a babysitter. Start with neighbors, church, friends. If you are friends with any teacher...elementary, etc. you can ask them if there was any student they had that they would recommend. (Even if they teach 4th grade, the responsible students will stand out.)
Then, arrange to have the sitter to your house for a "trial visit." Yes, you would still pay her. But...don't have a specific plan or time you need to be somewhere. Have her come over and help with the children while you are "getting ready." You can tell a lot by watching her comfort level with the kids. Does she get down to their eye level and interact with them? Maybe you and your husband could go to a nearby restaurant for dessert. It might be the baby's bedtime but the other two could stay up until you get home. (Plan to make it an early night.)
Also, set the sitter up for success. Have all the emergency numbers (and your cell number) listed and easy to find. Write out any special instructions and make things as easy as possible. (Have the kids fed when she gets there or at least have the meal ready for her to feed them.) Save a special movie for them to watch while she is there. Show her how to lock the front door and give her a tour of your house so there aren't any surprises. When I used to babysit, I had one family who bought me a special treat...candy bar or something....and told me to take it and put it in my bag and I could eat it if I wanted. (It helped b/c I used to worry about whether I was allowed to eat their food.)
Your 5 year old is old enough to tell you a bit about his/her evening. Trust your instincts. You'll know once you find a great sitter!! and...once you find her...treat her like a princess so she'll want to babysit over and over. :)
good luck