At 5 I find about half the parents stay, so if you think she would be ok then you will be fine to leave her and quite possibly will give other parents the confidence to do the same.

I usually stay for parties if they are a fair distance from home though, and if I have to take my other kids I just pay them in to the playcentre and buy their food myself.

Best thing wrt RSVPing is phone the mum who is hosting and explain that you will have your other kids. She will most likely respond that it's ok to leave your DD, or ok to include the siblings, whichever is her preference.