My toddler loves playing with the magnets on the fridge, but most of them are either too fiddly for his little fingers or too delicate for his indelicate play.
This project is a bit of a deviation from my usual long-and-involved tech-heavy Instructables, but it ended up being one of those simple ideas that just worked out really well. When I saw the "Three Ingredient Competition" it gave me the final impetus to share the project.
The TL;DR is: stick some magnets in some bath toys (tada!), but I will present a few thoughts I had on how to do it and how to ensure that they are toddler-safe at the end.
Step 1: Gather Ingredients
There are three ingredients
Find some nice bath toys that you think will appeal to your little person. Specifically look for ones that squeak, because they will have a handy hole in the bottom with a little whistle.
String Rare-Earth Magnets
The magnets need to be strong or they won't hold the toy on the fridge. I chose some 20x10x2mm NdFeB magnets, which I got from Aliexpress/DealExtreme, but you can probably get from Amazon or similar if you are feeling more rushed/less adventurous (expect shipping times of 6 weeks minimum from the Chinese "free shipping" sites).
I think it is important to make sure that the magnet is bigger than the hole, this way it is impossible for it to come out, even if the glue fails. You really don't want you toddler to swallow a powerful magnet, chances are he won't even develop any super-powers.
I spent a whole long while looking for a glue that was food-safe for my first ones and chose "Bostik Silicone Sealant" (I have attached the datasheet where you can see that it claims to be safe).
However, the glue actually didn't stick very well inside the toys, I would recommend using hot-glue from a glue-gun. I think it is pretty safe (but make up your own mind, its your kiddo), besides, its kinda locked away inside the toy.
Step 2: Remove Squeakers
This is self explanatory, but I took the pics, so it's a step now ;-)
Step 3: Insert Magnets
We purposely chose magnets that were bigger than the holes, so they can be tricky to get in there. I used a heat-gun (a hair-dryer would work too) to warm up the toys which made them a bit more pliable.
Step 4: Glue!
Try to shake the magnets to the base of the toy, then squirt a whole lot of hot-glue through the hole all at once (you can't go too slowly or it will set before you are finished) and then stick the toy down on something metal so that the magnets are pulled into place while the glue sets. I found that the glue doesn't really stick to the plastic, but if there is enough it forms a plug that locks into the internal shapes and prevents the magnet moving around.
That's it. Now you can put them on the fridge and your toddler can ignore all your hard work and go and play with something else like mine did.