Haven't read all responses, but just wanted to show the flip side of OP's point, our situation.
We're raising our children bilingually in German/English and try to speak as much German at home as we can, and I particularly 'pushed' German before they started school so that they had a good foundation before starting English.
Both my children (not Y3 and Y1) have learned English extremely well, and in fact are slightly above average in reading and writing than their monolingual English peers. They are also picking up French very quickly (perhaps because their brains are used to the concept of a thing having different 'names'?)
So many studies have shown that bilingual children do better academically, including at English than monolingual children.
I therefore feel so strongly that parents who are fortunate enough to speak another language than English, to speak that langugage to their children! They will learn English anyway and very easily. In fact, I am now struggling to get my two to answer back in German - so strong is the 'majority language' environment.
So please don't assume that speaking two languages hinders childrens' academic abilities - the contrary seems to be true .
Of course, what you may be concerned about is not the fact that these children speak another language, but that these childrens' families are not supporting them in other ways and that this may hinder their academic development - but that is an entirely different question imo.