If your girl prefers porridge without stock, maybe the stock wasn't to her taste?
There are many combinations to cooking stock, so you may experiment a bit.
I like to use sweetcorn + other ingredients (carrot, celery, etc) as it makes the porridge very fragrant and sweet.
My son is a big eater so far, I stop when he is not interested in eating anymore.
I read that force-feedng may backfire and let the child associate negative attitudes to eating. Better not do that as it is very hard to reverse any adverse psychological effects after that.