I think knowing that DP was going to get his tennis gear - he simply asked him to call in on Kate to see if she needed any help with the kids or whether maybe she had changed her mind about taking them out. Didn't DP report back to Gerry that she was OK and tried to persuade him to stay longer at the courts?
Considering the enormity of what happened later that night - I'm surprised they could recall such an insignificant event - (in comparison) - with any accuracy at all.
It's not insignificant when it was the last time somebody from outside of the family saw Madeleine. Trying to get Gerry to stay on and play tennis or it could be for another reason because Gerry left straight away
During the afternoon of that day the rest of the group members, including the children, were at the beach, [they] having returned at 18H30, the time at which he saw DP next to the tennis court. DAVID went to visit KATE and the children and returned close to 19H00, trying to convince the deponent to continue to play tennis, to which [entreaty] he did not accede as he had already been plying for about an hour and had to go back to to his wife. Nevertheless, RUSSELL, DAVID and MATHEW stayed to play.
http://www.mccannpjfiles.co.uk/PJ/GERRY-MCCANN-10MAY.htm