I'd not do it personally.
For a number of reasons
a) You don't know the dogs history and he came in as a stray so you have no idea of how he was treated etc by previous owner or how he reacted to things
b) JRT are known for being snappy and yappy. Many don't mean to hurt but will often try to grab a ball or stone the same time as a small child, catch LO's fingers and cause great upset and often rehoming or worse can follow.
c) The dog will need a lot of time to train, settle in, exercise (JRT are very lively and intelligent and get bored easily and are often constantly on the go) and trying to care for a young dog with a small baby will be hard work
d) If the dog does not settle or you are unhappy with it it will have to go back to the rehoming centre and face more trauma.
e) Its as important your child understands how to treat a dog as it is the dog to be trained with people and children. A 7 month old is not going to understand that if she drops something and the dog thinks she is playing that it may lunge toward her hands as they go to retrieve the object. Or if the dog yaps constantly (and JRT often do especially when wanting you to pick up and throw that sodding stone AGAIN for the 50 billionth time) it may bother her.
f) I'd be inclined to research your breeds more and look to finding a dog you have a more filled in background on and also a breed better suited to small children and a busy life. Shutting the dog away for hours on end or in a cage isn't fair and you'll need a garden you don't mind having holes dug in either. JRT are diggers to boot.
In their favour the one thing about JRT is that that are hardy and not really cuddly dogs at all so usually don't mind not having cuddles or being made a fuss of. But the down side to that is that if and when your LO wants to haul doggy along for a cuddle and dog does not want this..... dog may snap. End of the day, does not mean a bad dog (just does not have other ways of saying 'Leave me alone please') but it'll be the dog who is rehomed again and will be seen as the problem.
So yeah, thats my 2 cents worth
