A soft toy may be either hand made or mass-produced. In either case, taking photos of it to use in a children’s book is likely to raise copyright issues because the toy will usually be either a craft item in its own right (in the case of hand made toys) or a copy of a hand made original (in the case of mass-produced items). In each case, photographing the toy may mean not only that you you are making a reproduction of the craft item, but also an indirect reproduction of the design drawings and/or design sketches.
The safest approach, therefore, will usually be to get permission - or create your own soft toy starting from scratch so you know that you are on safer ground (provided you do not copy a “substantial part” of a toy in which someone else owns copyright).
Note also that, apart from copyright, this scenario may raise issues under areas of law other than copyright - for example, under trade mark law or “passing off” law. This would be the case if it could be argued that people reading the book are mislead into thinking that the book has a connection with the company that manufactures the toys.