Let's suppose you have two children and your U.S. Adjusted Gross Income (AGI) is about $75,000 or less. (If you earn more the math *might* change.) When you file your U.S. tax return (filing status Single, or Head of Household if you qualify), as a resident of Belgium (a comparatively high income tax jurisdiction) you should NOT take the Foreign Earned Income Exclusion or Foreign Housing Exclusion (IRS Form 2555). Instead you should only take the Foreign Tax Credit (IRS Form 1116). You should also take the Additional Child Tax Credit (IRS Schedule 8812). Follow that particular path, preferably using your favorite tax preparation software (even the free ones like TaxAct or TaxSlayer), and you should see a REFUND at the bottom of your tax return. Yes, the IRS will send you $1000 per U.S. citizen child per year in free money. Really. (In tax years 2009 and 2010 there was another $400 in free money available as a special refundable tax credit, but maybe you missed that.)
Take the money and save it for your kids, or spend it on your kids, or some of both. That's about $17,000 per child in free money over their childhoods. When they turn 18, THEY can decide whether they wish to terminate their U.S. citizenships or not. I'd advise them not (under present conditions at least), but under current law it's free to do so before age 18 1/2. Even if it's not free, they've started with $17,000 in free money plus interest.
No brainer, here: get your kids' U.S. citizenships documented. U.S. citizenship literally pays.