The notion the D90 is "now a quite old camera" is ludicrous. The D90 was launched just 5 years ago
High ISO in newer cameras has gotten better in recent years but for the OP's intended use, the D90 will do the job.
But, like with all photography, the knowledge and skill of the photographer plays the biggest role in determining the resulting image quality.
The OP needs to know to use continuous auto focus (AF-C), needs to set single point focus area mode, needs to set the white balance for the lighting type in the gym, needs to know how to pan and anticipate the action, and needs to know how to use the histogram on the rear LCD to judge exposure.
Since the OP has no experience, the season will likely be well under way before he learns how to effectively use any DSLR camera for shooting an indoor action sport.
It's also not likely the OP will have any post process image editing acumen he can use to 'fix' a photo.
So I recommend he put the D90 in green Auto mode and let the camera make the decisions. I think that will maximize the number of photos that have acceptable image quality.
It is worth noting that even professional action sports photographers using 10's of thousands of $$$'s worth of camera and lens expect some % of their photos to be unusable.
By undertaking a study of how to do action sports photography in between events, and practicing/experimenting on matches his daughter is not in, the OP can gradually take control of the camera and lens and accept responsibility for the resulting image quality.
Wow.. Keith, you know this is the first time I think I've ever disagreed with you on something. Not that I don't respect your opinion, I certainly do - but in my experience the camera's auto sports mode generally does a fairly poor job particularly in lower lighting situations. The camera tends to worry too much about keeping the ISO low and as a result the shutter speed drops to a level that really won't yeild much in the way of usable photos - just my experience mind you and perhaps the auto settings on the D90 are different than others but I've just never gotten good results using the "sports" modes on any camera other than in very good lighting conditions.
I'll agree with you whole heartedly on the AF-C and single point focus, white balance, etc - but I do honestly think they'll be better off going to A mode as opposed to using the sports mode in this situation. I think the learning curve for using "noise reduction" in post is a pretty small one, at least with most of the software I've used so I really do feel it's a better route. I do get where your coming from though, and yes as far as shooting situations is concerned this one is a toughy. Even pro's struggle with getting the right balance trying to shoot action in bad lighting.