Hi Trevor
Well I got to the record office today - some success, but not all I'd hoped

The only McGrath baptism in Rodborough at all between 1832 and 1840 was what I presume must be your Elizabeth -
1836 June 5th Hester Elizabeth dau of John and Louisa McGrath of Rodborough. John's occupation is given as weaver.
There was no baptism for Maria during that period at either Rodborough, or St Lawrence Stroud (which they might possibly have used if they moved further into Stroud). I also looked for a burial for John between about 1838 and 1842 but no sign (no McGraths at all) at either Rodborough or Stroud. I did think that, if John had died of an accident, it might make the newspapers but the Stroud ones only started up about 1850, so that's no good!
Since I've come back I looked up Louisa and the children in 1841 and notice that they were living in Wallbridge, which is about half way between Rodborough and Stroud centre, so either church would have been an option.
I must admit that I'm puzzled - I looked for Louisa' remarriage in both those churches as well but couldn't find it. If we can fin that, it's just possible that Maria's baptism and John's burial may be there as well.
As for Alexander, I'd be tempted to believe that he was Alexander John McGrath, in the way Elizabeth is Hester Elizabeth, but anything's possible!
I looked up the John McGrath from Cork in the Gloucester assizes records and don't think it can be your chap because his occupation was given as Tailor. However, since I got the details, you may as well have them!
Date brought in - 11 Aug 1835
Name - John McGrath
Age - 32
Parish - Coumty of Cork, Ireland
By whom committed - RB Cooper, JE Viner, J Newell esquires
Crimes - Charged on the oath of George Harris and others with having on the thirtieth day of June at Cheltenham feloniously stolen a Drab Great Coat, a Blue Cloth Coat, a pair of velveteen breeches, two shirts and other articles the property of the said George Harris.
Marks, Stature etc - Black hair, Hazel eyes, round visage marked with the small Pox. Can read and write. tailor. 5ft 4inches
When tried and event of trial - Summer assizes 15 Aug 1835. Not guilty
When discharged - August 19th 1835
How behaved - Orderly
If we can come up with any more ideas about what church might have been used by the family, I'm happy to have another go!
While I was going through the registers, I found quite a few Dark baptisms between 1832 and 1840, so if you'd like those, I made a note....
Best wishes
Angela