Your data is on the iPad - period.
If you delete the app your data will be deleted. If the iPad gets lost, your data is gone, if your iPad fails in some way then your data may be lost.
Can you get your data back from the Sport Ngin site? NO
Sport Ngin is a one way transfer (with the exception of Roster Players and game parameters) from the iPad to the web site. Sport Ngin does NOT store any details and only has the summary information about the statistics from a game. The iPad has all the original information to create that data. That raw data is NOT backed up in any way to the Sport Ngin site.
So please backup your data. We have had instances where school IT departments wipe the data off the iPad at the end of the season. Instances where the iPad was lost, and where the download of a new version of the app from the store failed due to some network issue. In all these cases the data for a season (or multiple seasons) was lost.
You can backup your data or export your data to preserve it.
To restore from Dropbox see this link: Dropbox Restore
You can also do a simpler step by just exporting your data to an email account and saving the email attachment. The disadvantage of this is that it is up to you to keep track of what date this was done and to physically put it somewhere for safekeeping (Dropbox by it's nature does that for you). To export a copy see this link: Export Data
The Restore step for the above export is covered here: Restore Exported Data