How to design workflow for an approval?

I’m using java sdk.We have biz process like this.

createBuyMenu()   //  this should be create by rest api, and **give a callback for who create this workflow**
makeApproval()     //  this should be auto start when createBuyMenu() finish
waitApproval()       //  this should waiting all approval finish.the approval maybe a **child workflow**.
transferMoney()    //  this should be auto start when approval finish

2 questions.
1.In my opionn I may call workflow in restApi.Then the first activity contains createBuyMenu should execute. But how to get callback here? Because I need create a response tell user createBuyMenu() Succes.

2.Approval must be reusable, so is child workflow is a best choice here.