We are in Brussels.
We rent the same size house as we own in the UK, cost is double what our house rents for, in a cheap area in UK, but cost is less than we'd pay for this house in a suburb of London. So housing is less expensive than London but more expensive than a provincial British city. We also have to pay almost all maintenance charges and we must give the house back in the same condition we rented it in.
Public transport is considerably cheaper. Hubby pay £400 a year for unlimited bus/metro/tram. Pay average of £80 a year for our over 12s, free for under 12s.
Opportunities out of school for the children are fantastic and generally much cheaper. Our children swim as their main sport, the eldest I pay £220 a year instead of £800 a year. I pay £10 for a half hour pony lesson, would not have even considered it in the UK. Massive choice of activities, too much to choose.
Child care is plentiful and cheap and accessible, nearly every school has 7am to 6.30/7pm care. We pay a maximum of £4 per day for this care at school. During the holidays, weekly day child care costs from £40 to £150 depending on what activities are chosen, basic child care or a week of pony riding. Residential child care is plentiful too. I paid £150 for our eldest to do a Mon-Fri multi-activity residential course this Summer.
Schooling is DIFFERENT. Private international schooling costs from £6000 to £25000 per year, depending on school. Public school costs roughly £400 a year each child in the French system, less in Flemish system. School is more learning by heart and exams from age 6, less arts / music / team sports (that's done outside school), but it depends on each school as some are more forward-thinking than others. There are also alternative "method" schools which would be in the private system in the UK such as Steiner schools, which are in the public system here. Standards are without doubt higher in public schools - our 13 year old does Maths at same standard as his 15 year old sibling in international school.
Taxes are VERY HIGH for those with just 1 or 2 children, much less for 3 or 4 or 5 children as you are given a tax allowance with each child. Housing benefit does not exist. Child tax credits do not exist. Child benefit is same as UK for 1 child, much more for 2+ children and is not means tested. We get 2 1/2 times child benefit here than we got in the UK.
We don't save any more money here than we did in the UK. However, hubby has no night shifts here and sees more of the children. The children have far more opportunities here than in the UK. We do not have druggies at the end of our street like we did in our nice middle class area in the UK (god knows what it was like in the council estates where we lived).
I wouldn't recommend Belgium with 1 child unless you are on an expat package and don't pay full local taxes and unless you are comfortable with local schooling - I don't get the point of coming to Belgium to put children into international schools as you then never really get to take advantage of Belgium.