I have just bought my son a second hand Spesh Hotrock 20 (6 speed grip shift).
It needs new grips, I don’t think you can buy the spesh ones.

What are my options? Buy adult grips and cut them to size?

You can buy islabike grips directly. They are great for little hands and have the right lengths for gripshift.

Spank grom grips are small lock ons.

I got some silicon foam ones off eBay for a few quid, winner

We’ve had a few pairs of these over the years and they’ve been great:
http://www.ebay.co.uk/itm/231114866535
Being BMX grips, they are a little shorter (125mm) and reasonably thin for little hands.

islabike grips

They look spot on

The only downside with Islabike grips is that they can wander a little bit, although the newer ones are better. We’ve always switched ours to thin lock-on grips as they are more predictable.

I’ve got some cut-down lock-ons you can have, they were on my lads Fun Fatty.

Lizard skins do some kids grips that are decent. The Spank Grom ones are great though.

Lizard skins – https://www.evanscycles.com/lizard-skins-mini-machine-single-compound-grip-EV240814

Spank – http://www.chainreactioncycles.com/spank-spoon-grom-kids-grips/rp-prod146077?gs=1&utm_source=google&utm_term=&utm_campaign=Chain+Reaction-UK-PLA-PLA-All-DT-SE-Shopping+QLB+Manufacturer+Desktop&utm_medium=base&utm_content=mkwid|s2NDPhzii_dc|pcrid|161845658788|pkw||pmt||prd|531372UK

cut down skinny cheap lock-ons here

Spank Grom grips are the same diameter (30mm) as every other grip so no advantage in the dimension that matters, they’re just shorter.

I just went with heavily reduced slim Easton’s from CRC for both my lads as I’ve just got rid of the crappy gripshift for 7 speed “clickers” as the boys call them. Sooo much better now that they can actually change gear.

those Easton slims aren’t that slim at 30mm