Hi @Kaysie and welcome!
It's not clear what you mean by "on the deal"? I'm guessing you're looking at taking an upgrade offer but then giving the phone to your grand-daughter, there's nothing stopping you do this.
As long as you realise an upgrade will reset your minimum term and if you find within the next 2 years that you need a new phone, it could get rather expensive!
You can use any EE SIM in any EE phone, providing it'll physically fit. Equally you can let anyone else put their EE SIM into your phone, providing it'll fit.
Doing either won't alter your account, contract or number - that's related to the SIM.
