Have you tried playing with cake dough yet? If not, you should! It couldn't be more simple and although the boxes of cake mix can be pretty expensive (a couple dollars or more) this dough lasts well over a year, so it is still frugal!
For a sweet My Little Pony messy play bin just get a box of strawberry cake mix and dump it into your bin. Then add the ponies and present it to your little one. They will be delighted!