Yes Beta Chip is the recommended substrate if you can get it. Alpha chip can also be used.
Some keepers find the recycled paper product called "Carefresh" is useable for Kenyans.
For most baby snakes I recommend paper towel to avoid all ingestion problems, but Sandboas are bit tricky to get feeding unless they are completely covered, and that means using some type of particulate type substrate, and of course they will usually ingest some as it sticks to their food.
I personally don't like aspen because I've lost snakes to it, through ingestion of the occasional long toothpic like chards that come in it. if they swallow long pieces of wood, they might die, if they can't pass it.
If you must use Aspen, make sure it is the finely shredded stuff with no "long sharp tooth pics"
The shredded aspen Americans get is much finer than the shredded Aspen, we seem to get here in Canada...
I've been keeping and breeding sandboas since 1986 on BETA CHIP... and yes they do eat bits, but I haven't lost a single specimen to impaction in all those years.
Rens Feed is on the North West corner of Trafalgar and Burnhamthorpe, in Oakville
Beta Chip is just under 10 bucks for a very big bag.
It looks like this