"Spill" is perfect. Yes, it is used commonly for liquid substance.

From [Cambridge Dictionary](http://dictionary.cambridge.org/dictionary/english/spill)
>**Spill:** to (​cause to) ​flow, ​move, ​fall, or ​spread over the ​edge or ​outside the ​limits of something: 

>>*I spilled ​coffee on my ​silk ​shirt.   
>>You've spilled something down ​your ​tie.   
>>Let's ​see if I can ​pour the ​juice into the ​glass without spilling it.   
>>He ​dropped a ​bag of ​sugar and it spilled all over the ​floor.   
>>Crowds of ​fans spilled onto the ​field at the end of the ​game.*

It's clear from the second and last two examples that whatever that was spilled, was clearly not liquid.

"Drop" is a good option, too. Suppose she has a spoon full of solid food and she drops it? 

From [thefreedictionary](http://www.thefreedictionary.com/spill):
>To cause or allow (a substance) to run or fall out of a container.

You can also say:
>Don't spoil your food.

or
>Be careful, don't spoil your food.