Regarding what PP said: its best to declutter and minimize your own stuff first before tackling kids toys and books. That already will be a big help.
I love looking in my wardrobe and drawers which now have space and only clothes and shoes that I really like, are comfortable and flattering.
Next do easy rooms like the bathroom/kitchen. Many of us keep way too many utensils, saucepans and cooking dishes.
Look at all the random make up/shampoo/moisturiser samples etc and ask yourself if you're really going to use them. If the answer is yes, stop buying more until they're all gone.
Finally, work out how many clothes and shoes your DC need and reduce it to that. Only keep the best as hand me downs.
Finally, with toys, it depends on ages. I did an initial clear out of things I knew were never used (party bag/happy meal trinkets). Then for other toys I roped the DC in and we sorted them together. E.g. thanks to MIL giving us DH'S enormous collection of little cars we had well over 300!
I got them all out and split them into categories. The DC found it very easy just to select about 60 that they love and play with.
Ditto with soft toys, lego etc. And books! We had dozens of science/dinosaur/space type books. When laid out together we chose which ones we wanted to keep and which were "duplicates" or unnecessary