Weddings are very stressful. You have to be on point every single moment. If this is something you want to enter into I'd look to go as a guest and take photos. Then you don't have the pressures of having to perform but you can at the same time build your experience.
You could also ask them to hire a professional, then perhaps offer to assist the photographer. Be a second shooter or even help them with equipment, flashes, and reflectors. This can really help you and who knows maybe the Photographer will ask you to come on a few more shoots with them.
I took on a wedding without doing this. I don't regret it but I feel it could have made things go smoother. I did months and months of research on the reception and ceremony site. I went at the times of day that we'd be there in various weather situations (rain, sunny, overcast). I brought my husband to do test photos.I had 2 Nikon D90's with kit lenses. I went to a local camera store and rented a portrait lens and lighting. Since I didn't have the money to buy at the time, renting was the best way to go. I rented it twice before the wedding over the weekends.
I had to figure out my settings for the various times of day, places to shoot, places not to shoot. I did a good job, they hired me off of the portfolio I had prior to though. I reminded them on numerous occasions about my ability and that this was my first wedding. That I'd prefer they'd go with a professional. I took engagement photos and couples photos. I couldn't go from family photos right into wedding photography. But the stress and the amount of pressure to get that shot is huge. Go outside and start taking photos of your children. Do it as if it was a wedding, rushed and there are no do- over moments. Sure you can pose them after but nothing beats in the moment emotion. It's fast pace and you just have to be sure you can perform under pressure, and produce quality shots with every click of the shutter button.
If they still want to go with you ensure you let them know about your abilities, stress to them that you'd prefer they go with a professional as you're an amateur. So long as they remember the fact that you are just starting out and have never done anything like this before, then I say try to take it on.
Remember one thing... wedding photos are the only thing a person has as a memory of their wedding day. Their wedding memories are in your hands. How comfortable are you with that should you not do a good job?
