Good cup shape - Beautiful design - but after 4 weeks by "gnawing"

I bought the mug 2 months ago, it is regularly used for about 1 month (child then 9.5 months old):
- The cup is to keep the child easy to grip and even
- Also the tilt while drinking & hold the cup with 3/4 water filling without parental assistance goes smoothly
- Completely leak proof he is not: is the cup upside down in the little chair / stroller, so quite trofen smallest quantities (in warm season rather not a problem); raises the child the cup to the ground but not incur any puddles
- Design-technically a very nice product

On a negative note I have the following:
- When you buy any utility / assembly instructions was; This has made me the product once "rest" 4 weeks, since for fixed pressing the valve (it's time to go) no liquid crossed on my test; only after a loose fitting the valve and test with a child, I found that DOCH liquid emerges - an explanation of the operating principle here would be helpful
- Although men child never duration drinking, was "gnawed" already after 4 weeks of the beak, so that has become a major of the 3 small holes; the cup is still available - but overall not endorse any other products from the same manufacturer

I wanted to order the larger version, but am now unsure ...