Book 2 senior citizens tickets separately
Add 2 others n 1 Child under another PNR and add the coach preference which you get in the senior citizens ticket, most probably you will get in the same coach.
Secondly, if many tickets are available under GN quota book all under one PNR and avail concession (if required) for senior citizens seats will be alloted from GN quota (Note: LB are not Guaranteed in this for Senior citizens).