Hi Lisa- I emailed you offline but didn't get a reply so i am trying this way. I don't really know anything about Lydias family so sorry to disappoint you but I have alot of research on her siblings so I can help you there if you want.
I am interested how you know the names of her children as all I could definitively find out was from the few baptisms I found. Presumably this was passed down through your family?

The LDS the the mormon church- the church of latter day saints and they have put many records online that you can search through- some are indexed- some are not. Most of the PEI ones are not though the PEI government have the baptisms online which has been a big help to me.

My John Nelder- first born 1815 in Plymouth( we think) son of susanna and Joseph went on to Ontario- the manitoulin Island where many generations of my family hail from.

As far as I know:

Harriet married John Robblee and stayed on PEI.
Elizabeth married David Cameron and stayed on PEI.
Samuel married Mary Maria Susan Sherman and went over to Salisbury, Westmorland, New Brunswick, Canada - many of their kids went down to the US-alot of various Nelders in Maine
Joseph and James died young
Susanna married John Locke and stayed on PEI.
Mary Ann married John Issac Myers and stayed on PEI.
and then of course Lydia.

I have seen some Nelders on Newfoundland and not sure who they belong to. I don't think it is anything to do with Nicholas's lot( josephs brother who married rebecca) and I thought it was only Nick and Joseph who came over to the "new world" from devon.
( obviously they could have been related somehow further back as Nelder is not a common name )

Most of this Nelder line come from Cornwood and Ermington in Devon( though there is another one of Devon with a few Nelders and a lineage in London as well- perhaps they were all related pre that period I don't know as I have not been able to trace back pre 1600's) I was just down on Holiday in Cornwall and we came back via dartmoor in devon so I got a chance to see the churches in both places- was interesting)



Unfortunately- I have many docs backed up and waiting to be transcribed or/and translated from the french in many of my family lines so I tend to flit from person to family and get caught up in other things! Please get back to me if you want any thing else on Lydias family and I would love to know more about what you have on her children as that may give us a clue to help each other- bye for now- kim