I suppose like all seemingly unenforceable laws (like for example marital rape, although obviously I'm not conflating the two issues) the law acts more like a state-sanctioned censure that seeks to bend societal opinion.
It is not unenforceable at all, at least to the extent that circumcision could easily be discovered by healthcare workers during checkups. A government could even implement such checkups for boys with the specific purpose of discovering circumcision. Parents could theoretically face a trial -- if, of course, the government is willing to go that far. Consequences could be very serious, even if it now seems highly unlikely this will happen.
If it will banned in the Netherlands by the time I have a baby son, I will gladly make use of the opportunity to enjoy a day in Antwerp. Or Düsseldorf.
Is there a serious movement in the Netherlands to ban it? That surprises me.