I forgot to mention the age difference... I wudn't want the difference to be more than 2 yrs... So, that the skills and power of younger one in all games could match with her/his elder sibling...
Assuming that one of my child is neither a prodigy nor a dull,

I want both kids of same gender... so that they can compete with each other equally in terms of power & choice of games kids play... Like if they go cycling together... Both have equal chances of winning... If they play cricket, both have equal chances of outsmarting each other... If they play basketball, both should have equal advantage in terms of height & stamina... If they are into craftworks, both will have nifty skills to match each other...
So, a girl for a girl and a boy for a boy will have better competition and similar choice of games...