It's important to keep in mind that no antifreeze product is without potential risk. There are several antifreeze products on the market that are labeled as low- or non-toxic. They are certainly less toxic than traditional antifreeze and usually contain propylene glycol or methyl alcohol, but they aren't truly non-toxic. Both propylene glycol and methyl alcohol have the potential to cause gastrointestinal irritation, central nervous system depression, and even death from respiratory failure in severe cases.
As a safer alternative, you might consider using sand rather than antifreeze to weigh down your basketball net. If you do use a low-tox product and your net base springs a leak, clean up any fluid immediately. And no matter the type of antifreeze, keep any unused product in a secure place away from curious pets.