I am trying to make sure I understand where you are coming from with your question... Are you concerned that your employees are going to get a tip you don't know about even if they provided good service and someone chose to tip them. Is this some sort of control issue to ensure they get no more $$ than quoted? If you mandate a tip, do you plan to pocket that or pass it along to the server/hostess who provided the services of the party?
I don't like automatic tips added to my bill. I do completely understand in the event of a large party because it just makes it easier to ensure the server gets a fair wage since they are missing out on other tables in order to provide good service for 1 large group. We belong to a local country club and the 20% is on each and every bill for everything but we understand that and it is not something that is hidden. All employees share in those gratuities. We still will occasionally give a cash tip for loading/unloading golf clubs, etc.
As for birthday parties, I agree with a flat rate for the party. If the hostess does a nice job working with the group, then that hostess deserves to receive a tip because the hostess is the one working the party, not you.
My daughter's bf was a host for Chuck E Cheese and he got stiffed all the time unless it was a Dallas Maverick, another well known person in that area or people who appreciate good service and tip accordingly.
Typically, I will tip no less than 20% . I tend to tip in cash (even if I used a credit card) so I know the server received the tip vs management taking it.
Another thing I HATE........ When a bill is presented and the tip is on the bill already but not clearly posted and then there is another tip line for the customer.. I see this as trickery by the place of business (not the server) to get double tips.