I think it's a shame that the first session does not allow the children to have a go on all four instruments and make their choice later on.
But as it doesn't, I'd play her some music featuring the violin and cello and see which she prefers. If there is no preference, I'd consider the practicalities.
Assuming she continues with the instrument, how far is it to the secondary school she is likely to attend and how will she get there every day? It will be difficult for her to to walk for a mile with a cello or bass on her back, as well as her other school stuff. It will also be difficult to carry a cello or double bass on the bus or train (as one of my daughters would testify)!
More generally, I'd say the pros and cons of each instrument are as follows:
Violin and viola:
Pros: Smaller and more portable
Cheaper (at least for the first few years)
Strings will always be cheaper
For the violin you only have to play in one clef (two for the viola, but you start with one)
For the viola you will always be in demand and it will sound less screechy at the start!
Cons: Many people find the playing position uncomfortable.
Will take longer before she makes a decent sound, partly because the bigger the instrument the more mellow the sound, but also because it's much easier to play in tune on a bigger instrument. She may be upset by that, especially if others in the family are less than encouraging!
Cello and bass
Pros: You can make a decent sound almost immediately
Some people find it a more comfortable playing position
Cons: more expensive, bulkier and heavier, cello has three clefs to worry about (but again you start with one). I'm not sure about the bass.
Violin and bass might provide more opportunities than cello and viola for playing in different genres such as jazz or folk.
It's probably too early to tell, but what sort of personality does she have? If she likes being the centre of attention, she might prefer the violin as there are more opportunities (in an orchestra) to shine. If she prefers to be more in the background, one of the others might suit her better.
Is she likely to be tall or short? The viola is lovely but (other things being equal) the bigger the instrument the better the sound. A short child/one with small hands is unlikely to play a big instrument comfortably so may end up playing one not much bigger than a violin. Of course all young children start on small instruments, but if you're a viola player it's nice to be able to move on to a big one eventually. Many female adult cello players don't play full size cellos, but (in my view) the difference in size doesn't make that much difference to the sound.
For context, I have one child who plays the violin and viola, and one who plays the cello.