Female Mouse biting Baby mouse?

You must never stick unfamiliar animals in the same cage. When you wish to add a new animal to your existing ones, you need to go through a process of safe introduction.

Pickachu and Lola lived in their cage alone, and have scent marked it as their territory. In nature NO animal would venture on another animal's territory like that. Tootsie came into the cage, and imposed on their territory. Pickachu is the dominant female, and is making it her duty to defend her territory from the intruder. Tootsie might be killed, she MUST be taken out immediately. This is not a case of aggression, it is animal instinct to react this way to an intruder in their cage space.

What you should was allow both to meet Tootsie OFF their territory, such as in a dry bathtub, table, anywhere they haven't been before. This lets them all meet without the need to defend. This process MUST take at least a few days long, or a week, so you MUST have a temporary small cage for Tootsie to live in until it's safe for her to move with the others.

Don't put them in the same cage. Tootsie could die. You may want to try returning Tootsie or Pikachu or putting them in separate cages. It's better to be safe than sorry. And if there's already been blood, you're headed in the wrong direction, and I'm sure you don't want tootsie to die. Don't leave them together and nothing bad will happen to Tootsie

What you need to do is take Lola and Tootsie out of the cage and let them get to know each other better, and then when tootsie is older but them back in with Pikachu because it sounds like Pikachu doesn't like Tootsie in her area and is going to kill her.

I have had this happen before I had to learn the hard way. So please take my advice. Because you will loose the new mouse.

You need to separate. Either take the aggressive female out and put her in her own cage, or put the new female in its own cage. Otherwise you may have a dead mouse.

I had the same experience with my hamster, she ate her child and when she spits her child out, it was in half... what we did was to separate the mom from the children or else there will be no children at all.. we don't want that do we?