What a great day out!
If you’re going from New St, don’t forget to check out the huge bull in the main concourse.
I guess your routing will be to Euston then across to Paddington but also don’t forget you can get creative and potentially (subject to ticket validity) go via Reading and come into Paddington or indeed if you like nice train journeys go via Cheltenham or Worcester too!
On way home time permitting and if your 4 year old isn’t bored then you could get off at Wolverhampton and get the Metro back to Birmingham Grand Central (New St) or anywhere else of course!
Otherwise I will leave others to comment!