Juanbablo your youngest will go to Kindergarten for a year and then be assigned the nearest school, or go directly to the nearest school, depending on when their birthday is and whether you decide to ask for them to do a year of Kindergarten to get their German up to speed.
Tbh I'm a fan of integrating properly but it will be really hard for your older 2 to be thrown straight into the German system and they will probably be very miserable for a year or so.
There is a grammar school type system in Germany. It varies massively by state, but in year 4 (not UK year 4, but the 4th year of "Grundschule" which they start at age 6 - very roughly the school year when they are 10 years old) they are constantly tested throughout the year. The results of the tests in German, maths and "HSU" (which is essentially geography/history/a bit of basic science like the water cycle) dictate which secondary they can apply for. If they have the right grades the school has to take them as long as its within area - it doesn't have to be the nearest.
I'm in Bavaria (see above) so anything else I say won't be generalisable, but the above is true, with the caveat that some states have "Geesamptschules" mentioned above, which are comprehensives and would make your life easier.
A former acquaintance of mine wangled her two secondary age children places at Gymnasium (grammar school) as guest students for their first year in Germany. This is sometimes possible for English or French native speakers because of the perceived mutual benefit - essentially they are foreign exchange students although they can live with you locally not a host family! However after the end of the first year they were essentially kicked out because their German hadn't reached a good enough standard and their grades were not good enough. However this might be the best thing to try for your eldest if you do want to use the native system - at Gymnasium there may be children willing to try to speak English to her or him.
A child can be booted out of the Gymnasium (grammar school) or Realschule (here in Bavaria, I'm not sure all states have Realschule) if they get a "6" in any subject (grades go from 1 - very good, down to 6 - fail) or 5s in 2 subjects on their end of year report. Here in Bavaria there is quite a lot of movement between schools - a lot of it is kids being kicked out of Gymnasium and joining their year in Realschule or even having to move down to the year below in Realschule. Similarly every year one or two children from each Realschule class fail and have the choice of repeating the year at Realschule or moving "down" to Mittleschule, which is the only school open to all students regardless of grades.
Mittleschule students do move "up" to Realschule often here - but again it involves repeating a year. A lot of children who go to mittleschule for year 5 work hard and get 1s and are allowed to go and do year 5 again at Realschule, and continue at Realschule. This happens again going into year 7. Its harder later as the subjects studied start to diverge.
I sent both my older kids to Realschule despite scraping the grades needed for entry to Gymnasium - this was there choice, and having tutored various failing Gymnasium kids put me off Gymnasium except for very high flyers. A lot of kids are tutored to within an inch of their lives and pushed and can't hack it, and have a miserable 8-9 years or get thrown out for failing anyway, which can't do anyone any good! From Realschule you can still go to Gymnasium for the last 3 years (sometimes repeating 1 year) or an equivalent to 6th form college (Fachoberschule) although I think this only exists in Bayern and Berlin.