Add all connected fields on submit

I have an 'parent' object and a 'child' object. Each parent has many child and each child only has one parent. I'm trying to see is there a way that when a new Parent item is submitted it automatically adds every single record from the 'child' object as a connected record?

I once saw something in the forum on doing this but for the life of me I can't find it again. 

Thanks

I have the same problem. I enrol a person on a training programme and want to also enrol them on all the associated modules that are part of that programme. In some cases I have 48 modules and I need to enrol them automatically as doing 48 manually is too much work for the user