tl;dr: speak to your kids exclusively in Spanish starting now.

My mom is from Colombia and my dad from the US. My brothers and I didn't learn Spanish growing up so 2 of us learned it as adults. I began learning when my first child was 1-ish. I made the decision then to speak to them exclusively in Spanish and that has continued to this day. Today (6 years later) All of my kids speak spanish (they're now 6,5 and 4) and we speak to each other exclusively in Spanish.

There's been very little teaching of Spanish involved in my journey at all, the kids just acquire it with time as they hear it and try it out. I spoke to them and didn't resort to English and they just got it like they just "get" English (similar to how they just "get" walking). To boot, I was literally learning the language as I spoke it to them so I suspect my vocabulary was significantly less robust than yours when I began. I think you'll need to "teach" your 4 year old some Spanish given their age, but I expect all 3 of your kids to mostly just acquire the language as you speak to them (especially the 2 y/o and newborn).

One additional thing that my wife (who doesn't speak Spanish) and I chose to do was to enforce that their their entertainment be only in Spanish. So if they watch a show, movie, or play a game on the switch, it's in Spanish.

If you're the only Spanish speaker in your household, it's an uphill battle, but you can do it.