That wouldn't follow the rhyme scheme of the other verses, which follow the scheme ABCC. The next verse is:
There is a lady all in white,
Holds me and sings a lullaby,
She's nice to see and she's soft to touch,
She says "Cosette, I love you very much."
The extra rhyme is unnecessary. Further, "toys" and "boys" are a very simple, sing-songy rhyme, out of keeping with the overall tone of the musical. That could be justified for this particular song (a little girl's fantasy of joyful childhood), but there's an undercurrent of despair as well that could be undercut by that kind of rhyme.
It's already a very free translation from the French:
Dans une maison pleine de jouets,
Où les petites filles de mon âge
Cousent les toilettes de leurs poupées
Et ne font jamais le ménage.
In a house full of toys
Where little girls my age
Sew the dresses for their dolls
And never clean the house
"Boys" isn't present there at all; "filles" is definitely just "girls". But they've introduced a slant internal rhyme into the line, and that is in keeping with the rest of the play. The key motif of the whole show is:
Do you hear the people sing
Singing the songs of angry men
The "ng" slant is in there several times, but I want to call attention in particular to the rhyme between "sing" and "angry men", with the stress on ANG followed by two syllables of lower stress. It falls in nearly the exact same place in the line as "toys" and "boys and girls". That's a much more sophisticated rhyme structure, complicated enough to make up for the trite rhyme between "toys" and "boys".
That structure isn't present in the original French, but it's in keeping with the rest of the tone of the play. Instead of Hallmark card rhyme, they've created a more intricate structure worthy of the (depressing) material of the play.