For experience you can either look at what is available in your area and offer as a volunteer there, you will probably need to explain why you want to do this and you will need adbs or crb or whatever check is needed. If you volunteer at more than one place you might need more than one. (One at each.)
You might need to pay for the check to be done since you are volunteering to help you, or they may offer to pay.
The best places (off the top of my head are):
Creches (some gyms, churches and work places have creches)/Toddler groups/play groups/pre school/nursery/primary school/Rainbows groups/cubs groups.
(I've tried to put these in rough age order.)
These would all be general volunteers but the other option is to see if you can create a specific opening for a volunteer for a set time, e.g. offer to help run a cookery or gardening club or something, share whatever skill you have. This might be in lunch time at school or nursery or something.
There are some clubs at weekends for kids, e.g. baby singing or baby gym, signing for children etc, which may be a good place if you work full time.
Lastly, one thing to remember is that wherever you work there might be nurseries/schools etc close by. Getting to and from them to volunteer on a regular basis may be easier than volunteering close to your home (unless you work close to home). If you did your volunteering one day a week in a lunch break it would possibly be easier than having to take lots of time off to do.
It's a good idea to think what volunteering might do:
Yes, it ticks a box with social services
But also it gives you a chance to see how you get on with kids, handle situations, etc
Also, if you can volunteer near your home you might make some friends who will be helpful to you once you are a parent (although this is not likely it is not impossible).
Good luck.