Schools
Unfortunately sending your kids to a private international school is always going to be the most expensive option. Many companies as part of the Expatriate package offer to contribute / cover costs related to employees families as part of the package.
Local schools are an option of course and the level of education is high throughout the country, but lessons will be taught in Flemish / French, so your children may find it hard at first. Most children will probably be able to learn French / Flemish relatively quickly so hiring a tutor in combination with having them attend a local school is a good option.
The state education system in Belgium is far superior to that of the UK, this is especially so in the Flemish sector. The Flemish education minister is very serious about ensuring all children in Belgium get an equal chance and that schools are properly funded. The teachers are respected, properly paid and as a result professional.
Another example, compare the average Flemish school dinner to the UK equivalent, there is no comparison, the Flemish government have spent bucket loads on ensuring the children get a proper balanced and healthy diet. They simply don' t do all this arguing with Jamie Oliver etc about the possibility of offering anything more than greasy cod and chips, they just get on and do it.
Note that in the North of Belgium (Flanders) that Flemish is the widely accepted language & although French is also the official language, you will not be made too welcome in many places enquiring in French so stick to your English!
Getting into the system
Registration is in fact quite simple, they will like to see, passport, birth certificate, proof of address in the form of a rental contract and proof of employment.
In Brussels 1000 you will have to queue for sometime to arrange all this, anywhere else, provided you have the paper work in order it takes minutes. You come back 10 days later to pick up the blue card - registration document - for which there is a small charge.
I can confirm that you can claim unemployment benefit if you move to Belgium, but you need to be fully registered with all the necessary paper-work in order to have this (5 year registration, passport, commune papers, rental contract, any official Belgian documents you may have). If you’re successful expect to receive in the region of €1000 / month.
Renting accommodation
Belgian rental agreements are HORRIBLE. 3 months notice plus 3 months penalty is what you pay if things don't work out, and you don't want to stick around for the 3 months just to pay the penalty. That is for the BEST available lease.
You cannot believe the amount of paperwork required so if you can let your company do all searching and the paperwork for you or look for somewhere you like and then let the company do the paperwork. For fully furnished, it is about €1000 - €1200 in a safe suburb.
It is also good to consider a suburb that has minimum 'foreigners', because the council gemeentehuis would process your visa faster. Your visa applications will be processed in the suburb that you live in.
Employment and other issues
Always check that you’ll qualify for the mandatory 1 month vacation in the first year. My company stated, after I arrived, that OOPS, since I didn't work in Belgium last year, I don't get a month this year. This can be standard and legal, so have them write in your contract if you want it. PLUS you get the occasional 3 & 4 day weekends- about the same as the States when it boils down to it.
Your work permit could take up to 3 months before you land on ground, including medical exam, federal background check, etc. These are mandatory for your work permit/residency if you are staying more than 6 months.
Settling in
Once you Visa is approved, you will need to attend a INBURGERING INTERGRATION course on how to behave like a Belgian. The course is really a waste of time: teaching you to take a bus, train, open bank accounts, how to use internet, how to make friends, etc. It is 30 Hours for foreigners who have work and compulsory by Law.
The INBURGERING INTERGRATION also involves the most difficult of Belgian customs - to sit in a run down yellow walled smoke filled bar drinking "pinchers" of Jupiler beer whilst talking about how grey the weather & how badly the national football team is doing ...if you past this test then you get your Visa guaranteed 100%!
Cars
You can freely drive your car with German registration plates for as long as you like in Belgium and in fact there is no way the authorities can find out how long you have been in the country with your car UNLESS you apply for a resident’s parking permit, in which case they will ask for the registration of your car. At this point that if your plates are German you will not be issued a permit and will be asked to change your registration.
To do this you have to declare your car and pay entry tax at Brussels Docks (costs between €0.20 - €5.00), then have the car pass a technical, then purchase insurance and once you have the Green insurance document you can then go to register the car. Here you will apply for Belgian vehicle registration and you need all the previous papers + vehicle ownership documents (with vehicle chassis #, owners name) which you submit and then after 1-2 days you receive your Belgian car registration document which replaces your original document (Pink slip) and you receive your new plates – simple. At this point you will also start paying your parking fines, speeding tickets & annual road tax but on the bright side you can get a parking permit.