I have nooooo idea about Petco. I very rarely shop there for food. The most important thing is protein content and the list of ingredients. I'll get back to you about the protein content (I need to look at my bags, but I think the blocks are about 22% protein, and the seeds are about 13%), but generally you want a food with a couple ingredients listed before corn. While I dont' think corn is a bad thing for mice (my two oldest boys are in great health and have eaten corn their whole life), it is generally just a filler in pet foods and adds little nutritional value. And most mice I know prefer not to eat it if there are big kernels in the mix, so you'd be paying for a lot more wasted food. If you have any local stores, Sunseed hamster and gerbil is another brand I've had success with, and a local store might carry it. I have a Petco really close to me I was planning on visiting soon. I'll take a look for you at what they have. You can also use a good quality bird seed as a staple diet, but that's not something I can be much help with.
EDIT:
I looked on Petco's Website. Out of all the foods I would recommend something like these (note protein content and ingredients lists):
http://www.petco.com/product/104152/Sun ... sterHPFood
http://www.petco.com/product/104151/Sun ... MiceHPFood
^^ These are both Sunseed foods with a good amount of protein for a seed mix. I've feed them before and my mice are ok with them.
http://www.petco.com/product/106131/LM- ... MiceHPFood
^^ I have never tried this food ever, but it does have the high protein you feed a block for and corn isn't the #1 ingredient. I think it would be a judgment call for you once you saw the bag.
http://www.petco.com/product/9192/Supre ... =RatHPFood
http://www.petco.com/product/9364/Supre ... sterHPFood
^^ This is Supreme brand. I've fed this before and also really like it. It has a lot of variety, but again you have to make sure they eat a little of everything to get proper nutrition. The Rat formula would be great for a seed mix to use with a block, and you might want to have a look at the hamster variety as being a nice compromise between blocks and seeds. It has 17% protein and might be a good way to just buy one food for your mice and have them get a little more protein like show type mice need, but not so much as if they were being kept in top condition. In your situation, I would personally choose this as a starter food and then adjust it. Besides, even if your babies are pet shop decedents, the extra protein is needed by growing bodies to be healthy.