First of all, how did Cookie get pregnant? Did you adopt her that way, or might be that Chessie is not a female but a male? Before you go any further, make sure they are both females. Sexing
http://www.guinealynx.info/sexing.html
Beside the grass hay (like Timothy), babies and lactating moms need a supplement, like alfalfa based pellets or/and alfalfa hay.
You have more options in supplementing that calcium: 1.either from pellets (like Oxbow
Cavy Performance), 2. or from Alfalfa hay (mix it with your Timothy, and you can keep the Oxbow
Cavy Cuisine pellets), 3. or last, harder to do though because of higher attention and knowledge, from veggies. Either plan you'd choose, will get them the extra calcium they need.
Be sure you follow the veggie chart very good
http://www.guineapigcages.com/forum/...arts-info.html , at least 1/2 cup of a variety of veggies twice a day per pig (two leafy servings, one non-leafy veggies). Your baby piggie will not eat that much at the beggining, but it will get there. Hay 24/7 (all 3 piggies), unlimited pellets for mommy (till she nurses) and the baby (till 6 months), 1/8 cup pellets just for Chessie (if she is over 6 months). The baby is able to eat solid (pellets/hay/veggies) from his/her first day.
You need to sex the baby
http://www.guinealynx.info/sexing_pups.html and separate it at at 3 weeks old if it is a male (he can get Cookie pregnant).
Be careful with the cage, it has to be baby proofed so the babby won't fall out or get stucked in between the wholes:
Baby Proofing - Guinea Pig Cage Photos
You can reintroduce all 3 piggies anytime (making sure as I said that Chessie is a female) but it has to be on neutral space. Put around lots of distractions, like veggies, hidies and so, and lots of space and see how it goes for a while (during floor time). Don't let them in the same cage right away till they don't get well on that neutral space. If they do, make the cage suitable in size for all 3 (2x5 grids), clean everything that they use, and that is all.
ps: Keep us posted on how things are going.