Unless any of those children have been sexually abused, yes, it is fine for a 5 year old girl and 4 year old boy who are cousins to sleep in the same bed. I would suggest though, that you discuss with her that *nobody* touches eachother underneath where a bathingsuit goes, and if anyone tries to do that, just to tell them that it's not okay to do so. If she tells you that she already has done that, then just remind her that we don't do that-- those things are private and then just move on... making too much a deal of it makes it more taboo.
Children who are sexually abused tend to act out, sexually, with other children, so if anyone has been abused, I would suggest they sleep in different rooms, and area monitored closely during play.
I hope that answers you question. Fell free to write back!
Sure, I'll opt-out and send this to Parenting experts.
This is a question that really depends largely on the values of the family, so I will just speak as to what is generally acceptable in our society.
Generally, it is considered okay for children that young to share a bed at a sleepover, even if they are of different genders. Once the eldest hits about 6 it's probably about time to look at placing her in a sperate bed from her cousin, but until that time there are few who would object.