Unspayed female rats are HIGHLY prone to tumors. Rats in general are highly prone to tumors. Money should have been saved from the start since you knew this.
I'm not trying to sound rude, but the "it's just a rat" attitude kills me. Would you let a dog or a horse run around with a mass hanging from it?
At 2, the rats are elderly, but they can live another year. My youngest boy developed a tumor in his final days, but he was already 3 and it was in a spot that did not impede his movement and did not appear to be causing him pain. He lived his final week fat and happy with his tumor because I knew for a fact his time as close. Luckily, my oldest boy lived to be 3.5 and had no medical issues.
Either way, the tumor isn't going to go away on it's own, and your daughter will have to sit there and watch a mass grow on her pet until it's too big for her to walk, maybe even bursts her skin. Either have it removed, or have her put down. Which will leave the other rat alone until she dies, unless you get two young females or neutered males.