You said it yourself! She is a baby, and babies would rather play than stop and go pee.
You are expecting way too much from a 14 week old puppy. When she is playing, you must stop the play and take her out at regular intervals, until she is older and has better control.
As soon she goes pee when you take her out, let her go right back to playing with her friend.
If she is so well housetrained at this age, she must be very smart and you've done a good job with her, but you mustn't forget that she IS only a baby.