How do yo go about starting an english speaking child in school in Flanders, Belgium?

If the kid isn't to old (like a toddler) just enlist it in a regular school... He/she will pick up the language very soon...
Remember, it wouldn't be the only child there that doesn't speak Flemish as a lot off the immigrants don't speak Flemish at home...
Besides, since they get their money depending on the number off children in the school, they'll be quite happy to enlist...
From the age off 2,5 a child can go to school if they are potty trained... The child can start after a vacation, that's to say, after the christmasholidays or easterholidays and the first off September. They can even start with going half days...
Remember that, once they are 6 years, they have to go to school!
When you arrive in Belgium, check out the schools in your town and go and talk with the headmaster, they'll explain everything to you!

Take up a petition..

There are a number of International schools in Belgium and Holland you just choose the one closest to you that caters for children of your age.

Found a site for international schools, found a few schools in Belgium that taught English in primary schools so hopefully that will help.

The earlier the better they learn the language, just go to a normal school with the child.

How old is your child?

If it is not too old, I would defenitely send it to a normal school. Maybe send your kid to a language camp during the summer and ask for extra help from the teacher Dutch. Some school might see your child as a waste of time, but other will defenitely be willing to help you further

My 4-year old only speaks English and is starting on 7Jan in a Belgian school. She will start her primary education the year she turns 7 and so has 1.5 years of kindergarten (kleuterklas) to learn the language.
Having spoken to the teacher I've been told she will pick up the language as she goes along. There will be 3 bi-lingual children in her class of which one speaks Russian/French and is learning Algemeen Nederlands. It's what children get taught in school. They don't speak or get taught Flemish in schools.
Any child up to the age of 8-9 will easily pick up new languages. Don't force them to speak the local language at home. They will do so themselves when ready.
Can't tell you on the technicalities of entering a foreign child into the Belgian education system though ... my child has Belgian nationality through me.
Good luck