The most important thing to do is to write a business plan to prove to yourself that there is a business opportunity that you can satisfy.
This needn't be too terrifying!
You need to work out what your costs are going to be. These will fall roughly into two camps - the money you will have to fork out to start with (i.e. buying kit, fitting out your space, etc) and then your monthly, regular outgoings - e.g. wages, rent, utilities, etc.
Against these you will have to work out your likely income. This is more difficult, but it will involve you working out likely levels of traffic and how much per head you will be charging.
Do your own research - work out how many comptetitive facilities there are near you, bearing in mind that what competes with you may not necessarily be another soft play centre, but could be, for example, a good, child-friendly swimming pool. Try and work out the size of market available to you - this will be quite hard I think - but you need to see if you can get an idea of how many people there are in your catchment area (which you also need to work out as well!) with children of the appropriate age.
Hope this helps - no doubt much more to think about!