Execute Out Of Process or In Process

HI,

I have a parent ssis package that invokes several child packages. One child package executes several times for one execution of parent package. If I execute child package in process ( Execute Out of process = false) it will give error "File is used by another process".  I can avoid this by set Execute Out of process to true. But after that it takes so much time to execute child packages. What's the best thing to do in this case?

Thanks.