It means that the kids ran out of the house with great velocity. When a seed pod bursts, it is not a slow and lazy looking process. It is like a gunshot, and the seed is propelled out of the pod with an amazing amount of kinetic energy. I linked below a video of a seed pod exploding. The film has been slowed down 16x, but the speed is still unbelievable.
The kids in the story have been cooped up inside all day. The air is hot. The air is stagnant, and the kids are bored. They have a lot of pent up energy.
They had had their tea, they had been washed and had their hair brushed, and after the long day of confinement in the house that was not cool but at least a protection from the sun, the children strained to get out.
So when the adult finally said that it was okay to go outside, the children ran out of the house with such speed and energy that they practically burst out of the house. That's what seeds do. They burst out of their casing.