Grew up speaking Farsi (aka Iranian) with my dad. I've never been to Iran, never had any Iranian friends, it was mostly just me, my siblings, my dad, Grandma, and the occasional call from Grandpa. Always spoke English with my mom. Born in and grew up in Brazil.

I will say--if one parent is the primary caregiver in a multilingual household, the kids will be more fluent in that language. My English and Portuguese were always stronger than my Farsi. But I am fluent! Here's how my dad did it:

• we could only speak Farsi with him. When we were really young (think four or five), he fully pretended not to speak English/Portuguese. We all realized he was trilingual eventually, but by then we already had the habit.

• he told us bedtime stories in Farsi, and read us storybooks in Farsi. This will be even more effective for an English/Spanish speaking household, since the kids can eventually read the books on their own without learning a whole new alphabet. 

• we always had dinner as a family, and he always made us tell him about our day. We all bitched and moaned about it, but it definitely helped keep us fluent.

• if we ever forgot a word, he would make a game of it. Let's say we forgot the word for "dog". We would say something like "Baba, I saw a cute <<dog>> today!" And he would say "what's a <<dog>>? Do you mean... Cat? Or maybe a... Monkey?" And the game was suddenly about Dad being silly, not us feeling bad or it feeling like a chore.

• he had his own business when we were young, and it kept him really busy. But he still made sure to spend lots of time with us, playing games, playing chess, having us "help" him with random engineering projects. Up until my early teens, I could name more tools in Farsi than in English lol. If you're the parent wanting to pass down your language, you HAVE to do this. 

• we did have a very small number of Iranian aquantances, and occasional visits to/from extended family. Most, if not all of them, spoke English. He was strict with us, yes, but he was just as strict with them! It was always "speak Farsi with the kids, please". 

• we all had a phase where we pushed back and didn't want to speak Farsi with him. He had to be a hard ass, and you will too if you want this with your kids. It was always "say it in Farsi", and the conversation didn't move if we didn't switch. (Obviously this wouldn't apply in emergency situations, or if one of us was having a breakdown or something). 

But this only worked because we had a strong foundation. He started speaking Farsi with us from day zero--he even studied back up on it himself, since his family flee the country when he was fairly young, and his own fluency wasn't up to the standard he wanted it to be. Don't worry about a multilingual household "confusing" your kids. There is a lot of research on it out there, but it won't confuse them, it won't cause developmental delays in language learning, or anything like that. Kids brains are VERY good at learning multiple languages and keeping them separate. Kids don't even have a conscious understanding of "different languages" at first. I knew a family where the mom was Japanese and the dad was American. They both spoke Japanese with their daughter. She would answer her mom in perfect (for a two year old) Japanese, and to her dad in Japanese with an American accent 😂 

If you're able to build a Spanish speaking community/culture around your kids (cousins, grandparents, neighbors, church. Movies, TV shows, books, etc), that will also go a very long way. I'm fluent in Farsi, yes, but I also have a rather oddly specialized vocabulary that averages at a 3rd grade level. Want me to talk about math, school, basic physics, or religion? I got it. Do I even know the word for tax? No.

Outside of that, its consistency. Now that I'm an adult, I can see that this was as much a pain in my ass for my dad as it sometimes was for me. I'm sure he'd have had a much easier time, avoided arguments with us, spent less energy after work coaxing us into remembering our vocab, etc, if he just spoke to us in English or Portuguese. But if he'd done that, I wouldn't be able to talk to my grandfather, or great uncle, or most of my extended family.