I got your issue but could not reproduce it. Could you please elaborate a bit more on your scenario how exactly are you opening the child window and which controls/elements do you use.
Also, could you please modify the attached project in order to introduce the issue so we could investigate it in detail.
On a side note, I just wanted to encourage you to take advantage of the support ticketing system (http://www.telerik.com/account/support-tickets/new-support-ticket.aspx) in cases when you need a prompt response to urgent issues. This is the best way to reach our support staff and attach sample projects.
the Telerik team
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