She'll be fine stella,my daughter was vegetarian at age three & I'd no problems at all,she was as healthy as other children her age & has had no issues with growth etc in fact she's way taller than me now.It is very daunting at the start but if her mum is vegetarian then she should know what to do,the one thing I'd add is make sure she gets enough iron,again was never an issue with my daughter but it can be,especially if she won't eat greens.Hope all goes ok,would you like me to come round & cook 😆 I'm not at home right now but I'll look through my cook books & see if I can find some things suitable for children xx