I have come across this situation several times and find it rather curious.
There exists both an informal and a formal imperative of 'aller' (as with any imperative), these being 'va' and 'allez' respectively. I would therefore expect a mother telling her child to get off the floor and stand up to say "va, debout". Yet I heard her say "allez, debout".
Why is she using 'allez' instead of 'va' with her little toddler?
"allez" is used here as an interjection. From Larousse :
allez !
interjection
Exprime l'encouragement, l'affection, la menace, etc. : Allez ! on se retrouvera.
It would be translated as come on! ("Come on, get up")
Should you want to use aller, a possible sentence could have been:
« Vas-y, debout ! »
or even, as allez as already stated is an interjection here:
« Allez, vas-y, debout ! »
Va alone is too literary to be used in that context but can be found in this famous verse:
« Va, cours, vole et nous venge ! » – Corneille, le Cid
