Currently have 15/32 but the gearing it to high, my daughter rarely gets to third unless she is on a road. I need better gearing for trail riding
I looked online and it seems that the size jumps a lot from 32 to 35 and then to 45 in sizes that I can find.
A 35 probably would be doable but a 45 would be to low? What length chain would I need for a 15/35?

Or would changing the front to a 12 be better?

Or do you have better suggestions?

Drop the front sprocket 1T.
The jump to the next available rear sprocket puts chain length into question, so the front is going to be the cheapest move. We have the stock output sprocket with the next size available rear sprocket, the chain is somewhat worn, but it is moved all the way forward on the adjustment, on a new chain, you'd have to add two links, minimum.

However, if you're only concerned because she's only using 1st & 2nd, I'd leave it be. If you're having problems getting the little machine to go on the trails you ride, I'd probably look at the various cheap power mods first, then change the gearing.

Power isn't an issue, I just want a better spread of gearing. I rode it over the weekend and went down the road and in 3rd I bet it will do 40 mph which is far too fast for the trails or her abilities.

Remember, dropping the front sprocket or raising the rear will change how frequently she'll need to shift and how quickly she runs out of first gear from a dead stop. You might be causing problems. OTOH, a new front sprocket is about $13, and 10 minutes of work to change, so I can't see any harm in trying it.
I'd say 40 mph is about right for a top speed with the standard gearing on the bike, my nine year old has had the bike nearly topped-out on some of the gravel roads we've ridden in the past.