I would agree with Michael. When my children were younger, most of the soft play parties we attended involved travelling by car.
If there were around twelve children, we would typically organise three cars, collect the children from their homes, drive to Beckenham Gambados, and then drop them back afterwards. Walking to several houses to collect children aged between three and eight, and then taking them to somewhere like Gambados by bus, would have been quite impractical. I don’t believe there is any real need to worry about buses becoming overcrowded.
If you look at the local swimming pool, which you might expect to have a similar impact, I’ve never experienced issues with children using local buses. The only real difficulty tends to be crossing nearby roads when swimming lessons are starting or finishing, as traffic can become quite congested at those times.