Hello Daniel,
The births you are looking for, John K Hill was born in the JAS quarter of 1913 in Cardiff, and Constance M Hill was born in the AMJ quarter of 1915. Arthur and Mary Jane hill who were married in 1901 in Cardiff appear to be the most likely family you are looking for. With her middle name being Jane it was quite common to use middle names at that time, also variations of a middle name. On the 1911 census the family are living at 46 Pant Bach Road, Whitchurch. It indicates that Mary Jane was from Perth in Scotland, so quite plausable for Jane to be Jean or Jeanie. The other children listed on the census are Frances Mary b 1901, Doris b 1904 and Lillian b 1906. All born in Whitchurch.
I hope this is some help,
Tallpaul.