Reducing working hours

First, you should make gross-net simulation on your salary (see here: http://www.references.be/tests-outils/calculateur-brutnet). Due to progressive taxation the difference in net is often lot smaller than you'd imagine. Keep in mind that reducing your hours below 75% will exclude you from some of the social benefits (f.ex. accrual of parental leave).
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If you are an employee with Belgian contract, and have been working with your current employer for minimum 12 months during the last 15 months, you are entitled to parental leave. The parental leave is max 4 months on full time for each child, but you can choose to take it as reduced working hours; 4 x 5months on 4/5, or 4 x 2months on half time. The leave has to be taken before the child turns 12 years.
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On parental leave your salary will be reduced according to your hours, but you will receive an allowance from ONEM (about 110EUR per month on 4/5 and about 400EUR on 1/2). If your child is born before 08 March 2012, you are still entitled for 4 months of the leave, but the allowance is paid only for 3 months.
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Slightly different rules apply to those working on public sector. More info in here: http://www.belgium.be/fr/emploi/gestion_de_carriere/conges_et_interrupti...

Comprehensive summary from AO.

Parental leave is a legal right - you cannot be denied it, but your employer can delay the start of it for up to 6 months (I think)
Nor can you be discriminated against for taking it.