1. Be sure that they like their
litterbox & litter! Very very important. It must be kept extremely clean to get them to use it reliably. Until they are about 4 months old, DO NOT USE CLUMPING LITTER! Use plain clay litter. Kittens play in their litter & can ingest quite a bit of litter from licking their paws. This will drop off as they age.
Take a look at the following for excellent articles:
http://cats.about.com/od/kittencare/
2. Next, please, please, please
do not declaw them 
It can cause a lifetime of problems. Learn to clip their nails, provide them with good climbing/scratching posts that are very very stable. This will give them an accepable place to scratch & stretch their muscles. The crappy carpeted post on a square base is worthless. Don't bother with Ebay either. The shipping is outrageous.
3. Feed them
high quality kitten food... this means NOTHING from the grocery store. Royal Canin, Felidae, Chicken Soup, Innova, and Wellness make wonderful dry food. Nutro is an acceptable brand that is easier to find. Do not feed cats dog food. Do not give them cow's milk. Many cats cannot tolerate it.
4. MOST IMPORTANT.... get the little darlings
spayed or neutered ASAP! The calico is a female, so she should be spayed by about 3-4 months. I don't know the sex of the Siamese mix, but I bet the same 3-4 month guideline will work.
FYI, my SIX cats have trained me well as to their requirements!
Good luck!