Some years ago I had a simple card /poster making programme for young children. I think it might have been a Microsoft one. It installed from a floppy. The user could change fonts, distort text, colour images etc. and print the results. It was great fun.
Now having grandchildren who are old enough to use such a programme, it has gone the way of all floppies.
Can anybody recall its name, and is it still available?
Don't underestimate how good even small children can be in using software.
In my view, buying "toy" software is not a good investment. It will be limited in what it can do and the child may then become frustrated with it.
I have two nieces who have been playing about with PSP and Photoshop for a long time now and, I'm quite embarrassed to admit that, they taught me a few tricks on Photoshop which I now use regularly. The younger one was five when she showed me.
Ho-Lin-Sok's link should provide some good software for your needs.