For schools, in some villages there is only one school, eg Nossegem has only one Catholic school, Everberg has one Official school, in bigger areas you will get a choice eg Zaventem central has 3 Official schools and 2 Catholic ones.
Catholic schools are traditionally lore sought after, but that is a general rule. This is because they are generally stricter, they cost slightly more (not so relevant in a Dutch area where extra costs are much lower than in the French system) and therefore there is a knock-on affect in intake, they feed into Catholic secondaries which are also more sought after. But that is a general rule.
In Official schools, religion is still obligatory and Catholic is nearly always the most popular option, Belgium is a majority Catholic country. In Official schools, there is always an alternative to the Catholic option for religion, protestant, ethics, Islam etc, the options differ according to where you live. At maternelle / kleuterschool level (6 and under) it is very rare for there to be any Religious lessons at all. But even in Catholic schools, religion pays a very very small part in daily life in 9 out of 10 CAtholic schools, they are not like Catholic schools in the UK, you do not have to be Catholic and there is no quota system reserving a certain percentage of places for Catholics. I would not discount Catholic schools at all, after all they make up more than 50% of schools and the percentage increases at secondary level.
I have no idea of the level of support in Dutch speaking schools, in the French system extra support is patchy. But do not worry in the slightest, in this area of Belgium there are loads of incomers, schools are used to children arriving without French or Dutch. Our children´s primaire school now has around 30 nationalities, even many Belgian children speak more than one language. Your children should adjust to the language quite quickly, especially given their age. Many Dutch speakers have excellent English, in French schools with international populations, there is always someone around or who can be summoned to help translate for parents. I know for a fact the Dutch schools in the areas I have mentioned contain English speaking children. If you were to move to Brussels, you would find even more English speakers in the schools, esecially the French ones.
Don´t limit public transport to weekends, it is plentiful in all but the smallest villages.
As for gym and dance clubs, seems to be plenty along the Zaventem - Leuven route, many are combined clubs, you will find many also held in the sports halls of the local school. Here´s a few I have found
Zaventem central
De Jeugdige Baabrekers
www.jbz.be/
Jazz Turnen Zaventem
www.jtc-zaventem.be/index.htm
Nossegem
Turnkring Nossegem
www.turnkring-nossegem.be/
Everberg
Turn en Dans Everberg
www.gymfed.be/clubs/clubdetail.lasso?id=472&m=4
Kortenberg
Turnclub GDO Kortenberg
www.gymfed.be/clubs/clubdetail.lasso?id=671&m=4
Link to Sports Clubs in Zaventem (includes the villages of StStevens Woluwe, Sterrebeek, Nossegem)
www.zaventem.be/94_nl.html