Will MDF stand up to the hammering a small child will dish out, MDF always seems so soft, especially around the corners/exposed areas.
Yes, MDF hold up to children. My grandson Odin, has a toy that you put balls in tight fitting holes and use a hammer to pound the balls through the holes. The hammer head is MDF. At this point it has survived at least two children. The hammer head is showing signs of wear but it still has lots of pounding yet to go. Note that there are many manufactures of MDF and even with the manufacturer's product line there are different grades.
You need to test the MDF to see if it is suitable for your intended purpose. In the case of my Hot Rod Freaky Fords. These are essentially solid blocks of MDF that have some holes drilled in them. Yes you can break them, but you have to work at it. I can't break one with my hands which I think might be a good rule of thumb. If you can break it with your hands don't use it for a toy. Of course it's not going to hold up to getting beat with a hammer, but neither will lots of wood.
Sharp corners are easily damaged. I have one sitting on my bench that I dropped on a concrete floor and broke a chunk out of the left front corner. A pine body would have been damaged but probably not broken. If it were maple the surface woulds have been marred where it stuck the floor. Odin has a complete set of Hot Rod Freaky Fords made from MDF that he has been playing with for about 20 months. There are some marks and dings but no serious damage.He has been know to throw them.
The rounder you make the corners the tougher they are. I sand the corners to a slight rounded edge because I'm going for the look in the plans. You could use a small round over bit in a trim router or mabe a 1/4 inch. You would need to try it and see if you like the look. Rounding over the corners can give a toy a completely different look. If you are painting with a brush I think the rounded corners are much easier to deal with.
Is inch and a quarter/inch and a half MDF available, or do we need to glue 1/2" or3/4" pieces together.
In my area MDF is available in 1/4 inch through 3/4 inch in 1/4 inch increments. It may come thicker but I've never personally seen it. To get stock for Hot Rod Freaky Fords you will need to glue up two 3/4 inch pieces.
IF it's a case of, "we need to glue pieces together" will PVA do the job, or are we better using Titebond III for everything.
Pretty much any glue will glue it together. I would not use Gorilla glue because of the foaming and its requirement for moisture to cure. I use Titebond II but any of the PVA glues should work fine. I like to use at least a water resistant glue for everything.
Conclusion
I feel I should add something about why I used MDF. Basically, I'm curious. In the True Colors scale I score as green as you can get. We always want to know why. I got a pile of MDF shelving nearly free and decided to experiment with it. I love experimenting. If I were going shopping today for materials to build toys MDF would not be high on my list. In my opinion material cost is almost irrelevant when making toys. At today's prices at Home Depot it costs about $6 US to buy enough S4S Oak to make one of these. There are much cheaper sources of Oak than Home Depot. If you making them to sell you can generally get more money for hardwood, maybe a lot more and it's easier to work with.
Working with MDF generates lots of very fine dust that is a big health hazard. You need very good dust collection and I recommend using a dust mask even if you're working out doors.
The bottom line is that you can make toys from MDF but I do not recommend it.
