I am a soccer coach for a local club who has the same policy - no jewelery. I hope everyone realizes that this generally is not a coaches decision, but a clubs decision and it is meant to keep everyone safer. I played soccer for many years, as well as coach - and totally understand this policy - could you imagine getting hit in the head with the ball and the back of the earring getting poked in the head?
It's unfortunate that you were not aware of this policy prior to getting her ears pierced. I remember having to really plan out when to have my daughters ears pierced in between sports - and still had it closer than recommended.
Like someone else said, everyone is different and when I asked the people who pierced her ears about how fast they would close up - they commented that everyone is different, but some do close very quickly. I would suggest that you take them out right before she gets on the field, have cleaning solution with you and put them back in as soon as the game is over. I would suggest maybe talking to her and explaining the situation to her so she is well aware and maybe letting her know that if they happen to close up (as that's a possability) that you will get them re-pierced when the season is over.