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The Will will only cost you £1.50 (I think), so not a huge outlay even if it tells you nothing. But have you considered she may well have been cremated?

It was also quite common for babies or newborns who died to be placed in a coffin with any local adult who died at the same time. You may be able to deduce where baby John was buried by looking at the burial register for other burials within a day or two of the baby's death, but I think locating the whereabouts of either child now rests on finding a document or record by absolute fluke. However the children obviously meant a lot to their parents and I think you can be assured they are buried somewhere in the Willoughton graveyard.

It is possible that the newspaper left out the word 'stillborn'.  It was not unusual to advertise the births of stillborn children and there would be no birth or death registered.

Debra 

That was my thought too.  I think your only faint hope is to find a family gravestone on the off chance that the stillborn girl is mentioned on that - perhaps buried with grandparents or the older boy who also died young.

Áine's mother, Mary Malone, formerly Connor, gives her age as 45, and the number of years she was married as 30 in the 1911 census, which suggests that she was 15 when she married.
In Theo's peripatetic addresses, I forgot to add one request to send his pension to Ballycotton Lighthouse, which he was painting at the time…
Here's Áine and Theo's grave in Glasnevin, with the Leeson Park Ave address; she was obviously a little discreet about her age at a time when marrying a man a couple of years younger than you was not done https://www.findagrave.com/memorial/249364350/aine-fitzgerald

Two points:  people before the twentieth century were much less aware and concerned about their ages than we are today, and my experience with my own Irish ancestors suggests the Irish were particularly unconcerned. Or maybe they thought they'd be economical with the truth on official forms just to confuse the authorities!!
Secondly the death notices and registrations were not done by the deceased themselves; if several people were involved, the notices were even more likely to be inconsistent.  The marble slab on the grave looks quite new, so whoever had it made may not have personally known much about Aine and Theo.