Is it in some sort of public service/voluntary sector environment? As well as being relevant in this case, some public/voluntary sector employers may be interested in experience you've had, eg if you have an idea of what it's like being a service user, or if you've done anything which would have involved talking to other parents (helping out with a baby or toddler group and/or welcoming new people).
I would go with mentioning it but make it relevant to any information like job description/person specification/what ad says they're looking for, even if it's something as simple as "I think my own experience as a mum would help me try and understand the needs of parents coming in to use the service..." (I'm sure you can word it better).
Thanks - I've taken a punt and put it in as I think it would help do the role. It's talking to parents about their children who have been diagnosed with something - so empathy from another parent is just what they need, rather than someone who can't imagine being in that situation.