Wyatt, my woodworking club makes thousands of blocks each year to distribute to needy kids at Christmas time. We use construction cut-off scraps that are donated. These are painted by senior center volunteers in non-toxic paints. It seems the kids go for the bright colors more than natural woods. I know, hard for a woodworker to understand.
Anyway, if you want to leave them natural, I would not leave them unfinished. They will get slobbery from the kids and stained and ugly. Maybe unsanitary as well. My advice is shellac. It's non-toxic, fast and easy to apply, and super blonde will look great. As for which would wood be good, the ones you mentioned are fine, as wood any domestic hardwood. There might be some argument for staying away from exotics that might be toxic, but with a coat of shellac I don't think I wood even worry about that.
If you do go with natural wood instead of paint, consider routing letters and numerals in the blocks to add a bit of interest.
I made a big set of blocks for my firstborn out of #1 pine, unfinished. The blocks outlasted the kid's interest in them (they lasted many years before rain and drool caught up to them).
I thought of using maple, but then remembered that all toys become weapons at some point, and decided to go with the softer species.
One thing I did that I recommend is to put a 1/8 roundover bit in a router table and round over EVERY edge of every block. This goes quickly in a router table.
I think I'd go with closed-grain woods, so there's no place for food boogers and bacteria to lodge. I haven't looked at ash in a long time, and I don't recall how porous it is. Maple would be good for sure.
Shellac is food safe (they use it to varnish candy, pills, etc.) but if you'll want to make very, very sure all the solvent has evaporated before Jeffie chews on the blocks.
Some other good finishes are ketchup, chocolate sauce and strained carrots. :)
Ash, maple, oak, cherry are all good woods for children's blocks. Walnut is a good contrasting wood. Most finishes are non-toxic. I'd pick a finish that allows for easy cleaning. Shellac is completely safe.
Personal Experience. I grew up on some Maple Blocks made from scrap. unfinished, couldn't have cared less about the color. the shapes were the fun part.
I was so intrigued by this conversation. I especially liked the person who said they make blocks and a senior center paints them to give to needy children. Anyone else involved in something like this? In particular, I would love to come up with something to do with all my scraps that are too small to save.
Check to see if you've got any smaller, more local, children's museums around. Our's is in dire need of some pretty simple things: a new Brio-esque train table, blocks, picnic tables, etc.
As a parent that takes advantage of these facilities, I'm working on giving back to them as much as possible.
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