Yes, the parents could be convicted of theft of the van in this case. To see why this is so, we must look at the definition of theft that is given in the link below. It defines theft as
A criminal act in which property belonging to another is taken without that person's consent.
What the parents did clearly fits this definition. First, the property clearly belonged to another. Just because the government is not a person does not mean that its property can be taken with impunity. Second, no one in authority consented to allow the parents to have the van. Finally, note that the definition does not say anything about how long the property must be held. The simple act of taking the property (which clearly occurred) constitutes a theft.
Therefore, the parents could be convicted of theft in this case.