Mr. Clark is right, and it wouldn't surprise me to learn that he knows more about this than I do, but I'll add a few points at which a spite registration application is challengeable.
To get a trademark, registered or otherwise, requires actual use of the trademark. That raises the ante a bit for the dad. So, it depends a bit how far dad is willing to take this. It's possible that any attempt by him to actually use the mark would be apparent to you. In other words, you can take some comfort as long as you don't see him using the marks. You might even threaten him with violating your trademark (unregistered trademarks are enforceable) if you see such use.
Next, for registration at least, the mark goes to the "senior user" -- who's been using the mark the longest without abandoning it. But, early in the registration process, that depends on the examiner finding your prior use.
You get your opportunity to stop registration at an opposition. Eventually, (assuming things go well for the dad) his application will be published and a period for public opposition is opened up. I have no idea how difficult (i.e., expensive) it is to file an opposition, but it sounds like that might be an end to the whole matter.
There's another thread here that goes into detail about all the pitfalls of using trademark registration as a tool for extortion:http://www.intelproplaw.com/Forum/Forum.cgi?board=trademark;action=display;num=1088739907
Another thing to consider is that the season will be long over before his application will mature into any rights that could be enforced in any court. So, you might tell him that you're more than happy to consider his trademark issue when he can produce proof of registration. Then, next season, use a different name. Game over for dad.
From the scenario you've given, calling dad's bluff might not be a bad course of action. Of course, that would require hearing threats and such for a while. Another option would be to let the privileged daughter join the team to warm your bench and field fly balls periodically.