If Kelly weighs 15% less than Megan, this means Kelly's weight is 100 - 15 = <<100-15=85>>85% of Megan's weight.
85% is 0.85, so Megan weighs 34 / 0.85 = <<34/0.85=40>>40 kilograms.
Mike weighs 5 kilograms more than Megan, so he weighs 40 + 5 = <<40+5=45>>45 kilograms.
In total all three kids weigh 34 + 40 + 45 = <<34+40+45=119>>119 kilograms.
That means to cross the bridge safely, they weigh 119 - 100 = <<119-100=19>>19 kilograms too much.
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