Model can resolve the sample time from base workspace only.
Just create the variable "sampleTime" in base workspace. In your mask, use mask initialization to change the value of "sampleTime".
For example: If you have edit dialog in mask and its name is mskChildMdlSampleT, then write below in maks initialization pane:
assignin('base', 'sampleTime', mskChildMdlSampleT);