Reddit reviews Still Quiet Place: Mindfulness for Young Children

Instead of laying with the 3yo, maybe your wife could sit in a chair nearby (could gradually move further every night if you want her falling asleep independently). You can also try coming in to check on her every 5 min, then 10 min, etc. Start with very short intervals that she can tolerate - even if it's just "Mommy has to go to the bathroom and will be right back" - and praise her for staying in bed. You could even do a sticker chart if she's motivated by that.

Sounds like she also may just not be ready to sleep - either overtired or not tired enough, not able to wind down, etc. Might be worth looking at the rest of the bedtime routine leading up to this and make sure it's as relaxing and low-key as possible. You could try some music or an audiobook/guided meditation (like this or this) while your wife is sitting with her, something that would keep her interest in staying in the room but not be so distracting it keeps her up.