In pediatric gait analysis the aim (hope?) is to be able to collect a trial with a clean strike on a pair of force plates with both limbs - this is always a challenge and if you get just one clean strike on one plate, then you have a useful trial. So I would suggest the second option, three in a row and one to the side at the start of the walkway.

This should improve the chances of getting at least one clean strike in a trial and if you are lucky, a bilateral pair of strikes. The two (left and right) plates at the start of the walkway allow you the chance to adjust the subjects starting point to get a clean left and/or right strike, while the three plates in a row give you a decent chance of getting at least one clean strike on one of the plates in a trial regardless of step length.