yes, you can buy mint flavouring. It tastes the same as the liquor
oh yes, way less. Average cake recipe for an 2X8' would be about 1tsp. What type of cake are you making? Can you post the recipe. That would help.
By the time you figure some of it baking out and then the rest of it being spread out among the batter there is not really that much alcohol in there.
An shot glass holds between 1 and 1.5 oz. A cup of liquid is 8oz. So you are looking at less than 6 shots in the whole cake before baked. I've baked with alcohol many of times and no one has ever gotten 'drunk, tipsy or anything' from the cake. And feel comfortable serving it to kids in my family (though I do give the parents a heads up.)
You use alcohol in a cake every time you use an extract. It isn't shots worth, but there is still some on it. 
. While I agree that most of the alcohol bakes out, if you really want to substitute extract, keep in mind that eliminating the creme de menthe will also eliminate a good amount of liquid and a good amount of sugar. I'd recommend making a simple sugar syrup that you can flavor with mint extract. For 2/3 cup of syrup I'd add 1 1/2 tsp mint extract.
and...creme de menthe comes in two varieties, white (clear) or green. If it specifies green, and your intention is to have a green cake, I'd add a little green food coloring tot he syrup. If it just says creme de menthe without specifying color, I'd assume green as that is the standard type, while the clear is a little less common.
While I agree that most of the alcohol bakes out, if you really want to substitute extract, keep in mind that eliminating the creme de menthe will also eliminate a good amount of liquid and a good amount of sugar. I'd recommend making a simple sugar syrup that you can flavor with mint extract. For 2/3 cup of syrup I'd add 1 1/2 tsp mint extract.
If it is a cake mix I'd just use the amount of water the box calls for. I've made a cake like this in the past with little chocolate chips in it from a cake mix.
Even if someone ate the whole cake they wouldn't be impaired from the amount of alcohol in it. And if they did eat the whole cake I think they'd have more issues with a full tummy and a sugar high than anything. 
The extract will work though you may have to play with the amount you add. Wilton and Loranne oils makes a creme de menthe oil that would be concentrated flavor and would give you a good punch of flavor as well.
If it is a cake mix I'd just use the amount of water the box calls for.
Why?
Creme de menthe is essentially a sugar syrup with mint flavor and alcohol. Why would you substitute that with mint flavored water? Sure it'll work. But you won't get the results the recipe creator intended from all that extra sugar...or that OP expects. There will be a definite difference. Otherwise why would the recipe bother to have you use creme de menthe at all? They would just say, add a tsp of mint extract to the cake mix.
I've made many cake mixes using alcohol and the same cake mixes with out. The only difference is the flavor of the cake. I, personally, didn't notice the alcohol adding any difference in the texture of the cake.
If she wants to make the cake using the simple syrup, fine, I was just pointing out that it can be done with out it with no additional problem. My thoughts is the creator of the recipe wasn't thinking about additional sugar in the recipe but more of using an alcohol they liked to flavor a cake.
I wouldn't normally use a simple syrup as my liquid in a cake mix. The alcohol is usually subbed in for some of the water, so if I wasn't using alcohol why would I do something opposite what I would have done to start with?
Again this is just my thoughts and experince with cake mixes and alcohol.
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