With burials in those years, it's unlikely to be connected with Crownhill RC as St Peter's didn't really get established until after that - see below.
https://stp-stm.org/st-peters-history/The Church of the Ascension (CofE) is also too late (consecrated in 1958), although the Methodist Church was earlier (see below) but no graveyard attached.
http://www.crownhillmethodist.org.uk/joom25/index.php/about-us/history/20-cmc-main-menu/about-us/history/69-a-new-chapel-is-builtWith the family living in Keyham, you'd expect any burials to be at a cemetery closer to them. Weston Mill or Ford Park would be the obvious choices. The latter offers burial searches for £10.50 (see below). Not sure if Weston Mill offers the same but you could try calling them.
https://www.ford-park-cemetery.org/index.php/cemetery-services/gave-searchesHave you tried searching on FindAGrave, BillionGraves and/or Deceased Online?
I also wonder if the children died in hospital rather than at home which may account for burials further away from Keyham. Brian Moseley has a page for hospitals on his 'Encycopaedia of Plymouth' website (now archived - see below) and it may be worth looking through these for TB and 'isolation' references.
https://web.archive.org/web/20131117020147/http://plymouthdata.info/Hospitals.htmDo you have any of the death certificates? As well as saying where the children died, you may find some clues about burial location. If they died in hospital, and the records have survived, you may be able to access them from The Box in Plymouth. These usually say if the deceased was buried in hospital grounds or elsewhere, and typically mentions their religion.
One other thought - you may want to look at St Budeaux CofE church which is on Crownhill Road. Devonport people sometimes used it for BMBs, and there are some instances of services being conducted there with burials then in Eggbuckland.