In a nutshell, you will be allocated a school based on where you live. Some areas will have specific 'integration' classes, which seem to be generally very good.
We live in Adliswil, on the edge of Zurich city, and it's quite a diverse area and they do have an integration class for primary school level, and additional language support for secondary school.
I'd second the recommendation also for Hofacker school if you can find somewhere to live close by in Kreis 8 of Zurich (finding accommodation with three children may be quite a challenge though).
It also depends how much effort as a family you will make to integrate socially as well - living further out might mean less foreigners, so you will get a reputation fast in the local area as either 'making the effort' or 'staying separate'. If you are also prepared (as the mum) to be active to learn Swiss-German and make connections, that can only help your children's success.
Your 13 year old definitely would have a steep learning curve - an Australian family we know relocated with their son at this age. He was a highly motivated, extremely polite and morally upright, self-directed young man. They tried a private school but found it was somewhat of a dumping ground for kids with learning and behaviour problems, so they put him for a year in the "real" schule level of the local system, which is where the intensive language programmes are, however the "realschule' is mostly populated by kids who have learning and behaviour issues, but the level of drugs and other antisocial behaviour was lower than the private school (more money seems to mean less parental attention and more access to drugs and alcohol) - anyway, that was his experience.
After one year, he was offered a transfer into the higher academic stream, but the parents found out that he would not get any more intensive German language support in that higher stream, so they opted to keep him in the Realschule level for another year.
After that he will do 2 years at a private Gymnasium, and then hopefully transfer to the Gymnasium of the government system.
A bit complicated ? Yes, but here in Switzerland there are many roads to get to the end-goal - and he's still a highly-motivated and morally-upright hard-working teen, who has a goal in mind.
It's probably fair to say that the 11 year old would most likely go to grade 5 integration class if available, and then maybe if lucky transfer to grade 6 of the local school and probably the middle level (Sek B) or possibly Sek A of the secondary school, if they can get their German up high enough. After 3 years of Sek they can start an apprenticeship, do a 10th academic year, or sit the exam (German, Maths, Geometry, French) to get into the "Short Gymnasium" - if they complete the Gymnasium and get a Matura, they have a direct path to university. If they don't do that, they can to a 10th year and then try again, or do an apprenticeship and sit the Berufsmatura exam (German, Maths, Geometry, French, English) to get into a 'Fachhochschule' which is the equivalent to an 'applied sciences' university.
It's also possible to try to integrate them for as many years as possible, and if they can't quite get there, to invest later (a lot of money!) into a private school for 1-2-3 years, if they truly want to get into a swiss university, and they are highly motivated...
Personally, you have to look very carefully at finances. I think for a family with three children and only one salary, that salary should be at bare minimum 80-90K, rent no more than 2500chf per month including additional costs, and you may not be able to afford a car. Spend whatever extra funds you have on saving a little for a rainy day and on the kids extracurricular, language tutoring and holidays back to see family.
We have three kids that age, and our food costs per month are probably 1200-1500 chf per month, and we all get a cooked lunch at work/school, which is not going to be possible with the local school (the 'lunch' care at my daughter's secondary school is 2 hours of care from 12-2 and it costs 20chf per day.).
I don't know if that gets you any closer to your answers, but honestly, accommodation is going to be the biggest hurdle with three children (especially if they all require their own bedrooms - we have a larger bedroom the two boys share and a tiny room that is used as an office/guest room so that we have more than one living area). - and it will really limit you to school options...