I am assuming this tank was cycled? If so, lets see if you can salvage the cycle. TAke the media from the filter. You can rinse in clean tap water that has been treated with dechlorinator. Rinse it only enough to clean all the sawdust and junk out of it. Set it aside in clear, treated water for the time being.
Take out all the gravel. Do you have a large plastic bucket? Put the gravel in the bucket and rinse it, dumping the water as you rinse. The sawdust and mealworms should, mostly, rise to the surface, unless it has been a long time soaking. Rinse and rinse again, until all the yuck is off the gravel.
Drain the tank empty and fill it again with clean water. Rub the insides with a rag, and drain it again - using as large a diameter hose as you have - so the sawdust and mealworms and such can go out with the water. Fill it again and do it again, until all the waste is out of the tank. I then dry out the tank by rubbing the inside with another clean rag. I like to set a tank on the kitchen counter beside my sink. That way I can use the sprayer to fill the tank, and a syphon hose directly to the drain to drain it.
You can now reset your tank. Fill it, heat it, set the air bubbler going. Put the rinsed media back in the cleaned filter and set it going. AFter it gets to temp, you can put the pleco back in the tank. Feed him lightly and test the water daily until you make sure it is still cycled. The cycle may bump - big or little depending on a lot of factors - from how much cycle you had to how much you salvaged. Having only the pleco in there will help a lot - for the cycle may be small now, but only one fish's waste is a small challenge. It can help the cycle rebound nicely.
When you see that your cycle has totally rebounded and is back to normal, you can consider adding goldfish back, one by one.
I am so sorry this happened. I keep all the fish food in tightly sealed jars in a top cupboard. When "small people" visit or I have a party, I often tape the lids of the tanks shut so no one can add a glass of wine or a cannister of mealworms to the tanks.
I also am a HUGE fan of sitting down and explaining to your daughter in caring, concerned words exactly what happened and why. She is plenty old enough, and I am sure plenty smart enough to understand a great deal of it. It is an educational experience - very sad, but educational never the less - and can be used to help her in the future. She may not understand it all, but she will see that you are sad and upset and that it involved a lot of work. She will understand that what happened was NOT good. IT will be a good lesson.
Poor fishies.....