Free font for children with reading problems

What free font do you think children prefer with serious reading difficulties (dyslexia, foreign language, low IQ, …). Children are from a Flemish Belgian primary school (Het Kompas, Gent), many non native speakers of Dutch (Roma, Turkish, Bulgarian). Most of them poor, some of them homeless, many abused or seriously neglected.
Preliminary indicative experiments and questioning point at Verdana > Source Sans > Roboto.
There is no budget and no possibility to prevent illegal copying.
Any help welcome.