I have found a note at the end of the entries for this church which explains the abbreviations:
NOTE:- Number of constituent members as far as known 41, whose manner of addition is unknown 40, baptized 965, experience 61, letter 272, died 501, excluded 110, dismissed to other churches 342, erased 59, removal unknown 21; Members March 1 1886, 252. The figures in the second column not otherwise designated denote the date of death of the member; b denotes baptized, e experience, l letter, ex excluded, d dismissed, + erased.
(I couldn't key the cross symbol so put a + instead - but that is the cross symbol you mention, so it doesn't mean death.)
So b = baptised but d would be "other".
Not found an explanation of the surnames and "Child" yet...