Multiple AVHD's, some older than parent.. need to merge? or safe to delete?

Hi,

I have a parent VHD and 3 children, the 1 AVHD file is current and working 100%, the other 2 AVHD files are exactly the same age and older than the parent respectively. I am very quickly running out of physical drive space and need to know if it is safe to delete the 2 AVHD's that aren't in use... I would attempt to merge them, however I understand that in order to merge the child files need to be newer than the parent. Any assistance will be greatly appreciated. Please see below:

Parent.VHD5/29/201358GB

Child.AVHD7/19/20131TB (Current)

Child.AVHD5/29/2013700GB

Child.AVHD10/8/2012370GB

Thank you.