Learning to read and write a 2nd language is the same as learning to read and write a 1st language. The 2 kids are not going to be in the same place, but you can find plenty of Spanish learning aids at teacher supply companies. Continue to speak Spanish at home and encourage your father to continue in Spanish. It's normal for your older child to rebel a little - don't punish her or criticize her, but keep speaking Spanish. If she responds in English, fine.
Buy some great Spanish language CDs for the car or the house, or for the computer if you have one. Music is a great way to learn another language because rhyming is so helpful. You can get a lot of famous books in Spanish - such as Dr. Seuss and many others. Familiar tales in 2 languages helps build a bridge. And learning a 2nd language, especially a Romance language, helps with English vocabulary later on. (Examples: green = verde, which helpss kids learn the English word "verdant"; esperar (in the sense of "to hope", not "to wait) helps with "aspire"); and I could give you 300 other examples without looking anything up!
Give them the Spanish alternatives of many things: Sesame Street, popular movies (many available on TV), books from the library, etc. Most libraries have a program that lets you borrow from other libraries in their network - you request and return at your local library. Just read to them as you always do.
Don't push the writing now. It's too early for the 3 year old anyway, and the 5 year old should focus on becoming proficient in her letters. It's the same alphabet for Spanish, so leave that alone. Don't worry about tildes and accents for now. The advantage to Spanish is that 99% of the words are spelled the way they sound, which makes it WAY simpler than English.
Also try Spanish cookbooks for kids - they can make simple foods, whether ethnic or "American", but follow the directions in Spanish. This way, you make Spanish part of their everyday lives, rather than a separate "subject" that will make them rebel. If you are of Mexican heritage, make Mexican foods for Dia de los Muertos, Semana Santa or Cinco de Mayo as well as Navidad. If you are of Puerto Rican or Dominican, Peruvian or Spanish heritage, embrace those traditions as well. Don't be afraid to invite your non-Latino neighbors or the kids' school friends to celebrate and cook with you.
It's great that your dad is learning English in terms of his own development, but it's wonderful that they have the experience of full immersion in Spanish language with him! They can teach him English slang, and he can teach them Spanish.
Don't worry so much about the confusion - young kids have an amazing ability to switch from one to the other! The earlier they learn, the more imprinted it will be. And they will learn Spanish without an accent if they learn before the age of 10, so keep up with it!
You will find that, in time, some of their friends will enjoy speaking a little Spanish with you - even if it's just a few words. So you will be the "cool mom" if you don't push it too much and just let it flow!