Possible Clues:

• Shooting up leads to lots of suffering for those developing while doing a stretch
• Increasing distress suffered by the young?
• Discomfort in the legs of children
• Expanding company's struggles, or the progression formed by the ends of 19-, 24-, 36-, 45- and 51-Across?
• Problems in the early stages
• Shooting up with 23 down produced early problems for those doing a stretch
• Tough things for parents and kids (2 words)
• Greece, in debt to Spain, possibly having teething problems
• Shooting up means trouble for those stretching their legs
• Problems that arise in the early stages of life