There isn't as much online for Belgium as some places, and what there is tends to be regional so it helps to know where they're from. Also, given the "Smith" (could be "Smit"/"De Smit"?), it would help a lot to be able to track down Helena's maiden name, perhaps via one of their children's birth certificates.
I see that in 1881 they have two children, both b. Manchester - have you been able to track them in later censuses?
Are the children consistently listed as b. Manchester?
Are the adults ever listed in any census as "foreign subject", "british subject", "naturalised british subject", or similar?
Do you have marriage certificates for the children? (if so, did they marry CoE, Catholic, other?)
Do they change first names/have middle names on any record?
Given the space between the kids it seems possible that either the older one is from another relationship or that there were other children who died young?
http://arch.arch.be/index.php?lang=fr_BE has a small amount of data online but I didn't see anything for your lot.
If you can find out where in Belgium they were from the familysearch site does have some belgium bmd images (not full coverage of the country, would need to know an exact location to search).
