I don't know if I would call this normal. Yes it happens. They eat the young for many reasons. first time mothers is one of the reasons and they all don't do it. most of mine don't do it. things to consider. are they all from the same line. (or at least raise up together) very important. not getting something out of the food you are feeding them. or not enough food. a nervous mouse in the breeding tank. you can make them nervous by too much cleaning of the tank,feeding, watching ect.... also they do better if you play a radio on low 12 hours while the lights are on or sunlight from a window, this helps with noise you make when you need to mess with the tank. the also need 12 hours of night, which needs to be noise free or close to it. I know a few breeders and the one who gave me most of this information had on average 100 litters per day of rats but he also breed mice. he no longer breeds rodents but did it for years. he owns the feed store where I buy my rodent block at. he supplies most the labs in my area.
If you do all thats above and they still kill the young, they become food for the snake. I produce about 100 mice, not litters per week, not per day. This works well for me.
good luck, don