Seized in this context generally no, I don't think it really means that they did not want to give it up- though they may not have wanted to. In a police investigation, usually means it has some significance to the case, or at least police may suspect it has and want to test it further for what ever reason. It has most likely been taken for possible evidence/testing. So while it doesn't mean the parents objected to it being taken- police could have forcibly removed it- in other words the parents did not or may not have had a choice...
Edited to add- I don't think the parents would have had a choice, (whether or not they were objecting- you would hope not if they are wanting LE to solve the case- unless of course it would implicate themselves in something!). Seized is used to say the items have been taken into police possession- which sounds fairly significant to me.