Imagine allowing your child to go on a field trip to the zoo, only to later find out that your child was bitten by an animal. Who would you be able to blame? Would it be the zoo's responsibility to keep your child safe or the school's job? You signed a waiver for your child to take the trip, so does it apply now?
Schools and businesses both have the potential to be held liable in the case that your child suffers an injury. Waivers can affect your ability to make a claim; however, they aren't always enforceable. For example, if your child was injured as a result of typical play on a playground and you'd signed a waiver, it's unlikely you could sue. However, if your child was hurt because of negligence or because the equipment was in poor repair, then the waiver may not apply.
In the case of somewhere like a zoo, it's pretty well-established that the zoo needs to be in control of the animals at all times. If, for some reason, an animal escapes and bites your child or is allowed near your child in a reckless manner, then you're in a position where you may be able to make a claim.
In the case that a volunteer or school teacher allows children to get into a dangerous situation against regulations, you may also have a case against the school for failing to protect your child from harm due to negligence. Your attorney can help you understand if a waiver affects your ability to file a claim after an injury.
Source: FindLaw, "Who's Liable for a School Field Trip Injury?," Andrew Lu, accessed Aug. 21, 2017