Take video proof with the client's phone, not just photos. Get them to film the extraction process so the evidence lives on their own device. Have them sign the work order acknowledging the toys came from their toilet before you even touch your tools. That timestamped recording kills any argument later.
Wait, is all that really needed? I mean most people are gonna admit it was their kid's toy once you pull it out. Idk, maybe it's just me but filming it on their phone and making them sign papers before you even start seems like overkill. Like the kid usually confesses halfway through anyway.
Actually you'd need to pull the toy first to know what you're signing for. Better to film the extraction, then have them sign right after while the evidence is still on the floor.