Hi! I don't have any children yet (well... one on the way!), but I have worked with diverse second language learning populations with a huge range of ages (and in some cases, special needs). Right now, I am employed with an international bilingual school (both English and Spanish are equally emphasized), but there are also several other nationalities and languages represented (Taiwan, Brazil, South Korea, Israel, and Switzerland just to name a few).
Some of the students come into this school system with little or no knowledge of either language, but because the school's environment is so "safe" (in the sense that they take a strong stance on bullying and emphasize the kids' taking pride in all their abilities, academic or otherwise), I've seen students come out soon after with 3+ languages as a result. Keeping in mind that the group of kids that I work with in particular are between 11-14 years old, I know it's not an exact comparison, I can still say that the younger a child is introduced to a target language, the better, but it is never too late to get anyone started on a second language! Honestly, I started learning Spanish as my second language a little later (13 years old) in a really superficial, non-authentic way, and although I've always loved it, I really lament that I never had exposure to it at a young age.
Especially if you are dealing with very small kids like yours (around 1.5 - 3 years as you mentioned), if a language or any new material is introduced in a fun way (in which they feel more like they are playing and laughing and enjoying themselves), this is really a great time in their lives to acquire (not "learn") the language naturally. Totally invaluable!
Since Spanish is probably not spoken in your home, I would strongly recommend a full-immersion situation in Spanish. The most important thing to look for (in my opinion) is that the staff at the school is kind, nurturing, and fun. If you and the kids are comfortable, it's a strong recipe for linguistic success.
Best of luck!!!