My children's toys are for only them and I, now, never let them share even if they want to.
When I did let my children share toys a DCK broke my DC's brand new toy and it was on purpose
My DC was so sad...broke my heart. I asked DCP to replace it (only time in 10 years I've had to do that, usually it's my own cost for wear and tear) and told my children no more sharing ever.
That must be very hard your DC have to share their stuff. Would it be feasible to get some inexpensive tall shelving and bins to store the more precious toys in?