I think that principle when one parents speak one (i.e. native) language to the child is a good one. I speak only Russian to my kids (ok, sometimes English) and must say their Russian is very good. Besides we read Russian books and watch cartoons.
I don't know how it's gonna evolve in the future, I have a few concerns. One is that in English school (I have noticed with my daughter) older kids tend to speak only English, even if, for example, both of them would be
Germans. It's all about what is cool for them. I also know a few examples when kids with Russian heritage don't want to speak Russian with their parents, cause they think it's uncool. Spanish is always cool, so you don't have to worry about it!
It's great that your child understands/speaks both Spanish/English. I'm sure that your kid will figure out German, kids are good and quick at languages, it's a fact. I moved here when my boy was just 2 and he learned German pretty quick. He was in English kindergarted in South Korea before. My both kids mix languages a lot. I've noticed when they play it's either in German, or in English. For 'serious talk' or with me they use Russian.
It also depends on the situation: if you are not planning to stay in
Germany, than you probably don't mind if your child knows just enough language to converse. We moved here for good, so I think it's really important that my kids learn German well.
It would be nice to meet up with kids, I'm available on weekends. We stay at Blakenese, tehre are a few nice playgrounds here. Let me know what time and date suitable for you.
Leyli