Welcome from me too

There is quite a detailed tree on Ancestry for this line (see
www.ancestry.co.uk/family-tree/person/tree/10305252/person/322085611774/facts). Not sure if you have a subscription there to be able to access.
Apart from Elizabeth, there was potentially at least another three children for William and wife Keith Ross. There are no entries showing for them in the birth & baptism database on Scotlands People. Being pre registration in Scotland (from 1855) lots of reasons why there are no entries showing.
William Allen and wife Keith Ross look to have lived past the start of official registration so you will be able to confirm their parents' details from their death certs on SP. The dates I seeming on the a/try tree for them are:
William Allan
Birth 26 JUN 1781 Cairnbrogie, Tarves, Aberdeenshire
Death 7 DEC 1862 Tillyhilt, Tarves, Aberdeenshire
Kieth Rutha Ross
Birth 12TH JULY 1778 Keithhall, Aberdeenshire
Death 17 JAN 1872 Tillyhilt, Tarves, Aberdeenshire
Always good to verify details for yourself that you may find on family trees if original certificates are not included. There are a number of original certs included though that you should find helpful when looking at the family.
Some burial details here for others in the family
www.findagrave.com/memorial/204868176Monica