Generally speaking, people with kids have higher costs- grocery bills are higher since there are more mouths to feed, possible childcare costs, increased electricity and water usage, in many cases higher rent due to needing a larger house with more bedrooms for the kids, increased clothing costs, higher medical premiums, and so much more.
If kids weren't included in the stimulus, many parents would have a much harder time paying bills, feeding the family, etc. If I was just buying food for me for instance, $1000 could buy me almost a year's worth of groceries. Add a couple kids into the equation and that $1000 would only buy food for maybe 3 or 4 months. $1000 doesn't go nearly as far when it's for 4 people vs 1.