The other thing I forgot to mention about Ixelles, would apply to other communes nearer to the centre, is that parking can be a problem and will get worse. Even if you get a garage (which you'll pay more for), it might only fit a tiny car, you might run into problems with people parking in front of the garage too. There is a policy of removing parking spaces in the centre areas and replacing capacity with the bike hire called VILLO and car hire called CAMBIO, in an attempt to reduce congestion.
If you prefer the early 1900s maisons de maitre to the 1960s onwards style you get in the suburbs, I'd look east possibly to Etterbeek, parts of WSL, WSP and Schaerbeek.
The area around Montgomery / Mérode is good, it's over those 4 communes just mentioned. It is also likely to be cheaper than Ixelles but with similar maisons de maitre with small gardens, on 4 floors. There are excellent secondaires and primaires there, but I can't emphasize enough you should choose the 1st or 2nd nearest primaire and secondaire of type if you are after a particular secondaire. I'd say with a year to choose, I'd get the school places, then work out the addresses where the primaire is 1st or 2nd nearest of type and the preferred secondaire too. In Belgium, you choose your own schools, limited only by finding places. Dérogation system does not exist. 40% of primaire children in Brussels go to Catholic schools, 70% of secondaire children are in the Catholic system. There is a definite bias to Catholic schools, with some exceptions, at secondaire level the following non Catholic schools are also popular in the area you are likely to be looking at : Athénée Communal Robert Catteau, Athénée Royal Jean Absil, Lycée Emile Jacqmain, Athénée Communal Charles Janssens. Robert Catteau and Emile Jacqmain are arguably the most sought after non Catholic French schools in Belgium (would be like going for places at Ecole Alsacienne in Paris), they are also highly academic ones, so you these schools really would have to be nearest school of type and your child be going to the nearest primaire of type to guarantee a place at the secondaire.
There is no college/lycée division at secondaire. Children stay a year extra in primaire until the year they turn 12 years old, then from 12-18 years they do 6 years of secondaire, divided into 3 cycles of 2. They cannot normally move within a cycle, so sign up for 1ere secondaire and you're there for 2 years minimum. Doubling is normal here too, over 50% will have doubled a year by the end of secondaire in Brussels. At the end of 2eme secondaire, the curriculum becomes more academic/technical/professional divided, some schools do only academic or technical/professional. If you know your child is not academic, it's a bad idea to send them even in 1ere secondaire to choose a highly academic school, as they'll only end of being thrown out by the end of 2eme secondaire. So if your child is not top 20% ability, don't even think about secondaires like St Michel, Emile Jacqmain, Robert Catteau. You should bear this in mind even when choosing a primaire like don't choose St Michel primaire unless your child is dead brainy and can handle their marks being read out for all the class to hear! Some primaires do exams twice a year, some primaires do only the compulsory national tests in 2e,5e and 6e primaire and less rigorous testing and no reading out of your results in public. You really need to ask plenty of questions about how children are assessed in primaire, as it varies so much from school to school, even though finally they are all taught against the same national French curriculum The 6e primaire exams are ultra important, but don't worry too much pass rate is 90%, fail and you either stay behind in primaire and double the year or go to a specialised secondary class where you'll be with those of lowest ability.
Maybe 40% of secondaires have an attached primaire or one nearby which is affiliated to the secondaire (in Belgese, it's called "adossement"). It used to be the case if you were in primaire X, you got a guaranted place in secondaire Y. This is no longer the case. The current rules state only those enrollend in primaire X on 1st September 2007 get a guaranteed place in secondaire Y, so this year's 6e primaire children who have been at the same school for 6 years will be the last year to benefit from this adossement plus next year anyone who has doubled and been at the same school 7 years I assume! So don't choose a primaire X thinking the adossement system will get a guaranteed place at secondaire Y as your children won't be able to benefit, without a change in rules, which I doubt will happen, as this adossement system is unfair to the majority of children who attend primaires where there is no adossement to a secondaire.
You'll have to bear in mind that most primaires are in fact fondamentales, which is combined maternelle and primaire schools. It means most children will move from 3e maternelle to 1ere primaire in the same school, so it will be just as hard to get a place for your younger child as your older one. Schools might also not be able to confirm school places until around April for you, they'd be able to give and indication at enrolment in say the September of the likelihood, but current children's parents don't generally get asked if they are re-enrolling for the next school year until January-February time. For example, if the 3e maternelle classes are full, they won't know if there is a place in 1ere primaire until all parents have confirmed their intentions for the next school year.
I have found it very hard making friends with Belgian parents. There are many nationalities at our school, even the French parents seem to be more willing to talk to the other foreign parents. Belgians I find pretty unfriendly on a deeper level than "bonjour", very family oriented, friends kept from childhood, see no need to add any extra friends to their lives. You might be more likely to make friends with Belgians through hobbies or your children's hobbies, as parents have more time to stop and speak than in front of school during drop-off. Work is unlikely. Some schools have a strong PTA and that was you could make friends perhaps. As for non Belgian ones, I bet you'll find much easier, inside and outside school. There is the Brussels Childbirth Trust (BCT) with 1200 members. There is a British and Commonwealth Women's Club, American Womens's Club etc etc.