It's a fact that children learn through play. Language learning is no different. Really make bilingualism as fun as possible for them.
That means making it emersive. Right when they are babies, and they are hearing mom and dads voice. Speak to them in the language all the time. Read to them all the time. Find them play dates and playgroups. Playgroups are actually really important in the life of the bilingual child for the context and culture. They need to have friends that are actually speaking the same language they are because mom and dad aren't always going to be cool. They need to know that their friends are kind of like them.
Travel, if you can, that's one of the best things you can do; especially if you have family who live in another country. They can also supply the language and the concept of love to the language. It needs to be relevant.
I always tell my daughter, when she starts speaking too much English to me at home, "You want to speak to your family the next time we go to El Salvador? Catch up on your Spanish because that's how we are going to do it."
Also, they love to watch movies. They can watch movies and TV at home during their monitored screen time, make sure they are in Spanish. There are Spanish apps. Anything they love to do, if there is an option in the second language, do it.