Hi bigjoe. We've done 11 cruises over the last 10 years, the last 8 with MSC. MSC, in my opinion, offers the best value for money. However, over the years, cruising has become very popular and prices have crept up as standards have slipped somewhat. By standards, I mean quality of food etc. Previous posters have given you good advice. I would add the following;
1. Most cruise lines add an additional Daily Service Charge to your on-board account. This replaces the traditional tips/gratuities which were discretionary. The charge is approximately €10 per person per day. It would apply to your children as well. MSC now include this in the fare. It is possible to remove or reduce this onboard but that suggests stiffing the crew. As it is added to your account, it is not necessary to tip additionally on-board.
2. If you like a drink, you may wish to purchase a drink package. MSC drinks packages are very reasonable at min €29 pppd. All individuals on your booking must have the same package but the 13yo only needs to have the soda package. If you go without a package, be aware that drinks on-board are expensive and water is only free in the buffet.
3. There are different levels of stateroom, from inside to balconies/suites. Better locations and staterooms obviously cost more.
4. Dining is a subject that divides cruisers on all lines. Food is quite good but not gourmet unless you pay extra. Dining times are usually fixed and you will probably be required to share a table with strangers but they are usually English speakers as well. MDR dining times are early(5.30pm on) and late(8.30 on). You will be allocated a time on your booking but you can change this onboard, subject to availability. This in in the main dining room(MDR) only. If you want more flexibility, the buffet is open at least 20 hours a day. You can sometimes book a package which allows flexible dining times or go to a specialty restaurant for a fee.
5. Most cruise lines have children's clubs onboard which you can sign up your child for. The 18 year old obviously not.
6. Ports of call are the big attraction in the Med. Availing of ship excursions is very expensive. Most tours can be sourced locally in advance via the internet for half the money. Don't be alarmed by scaremongering about missing the ship if you're on an unofficial tour, these tour companies are well practiced at getting you back on time. Larger ships often have to dock away from the city centre and ships will offer you a shuttle to the city but it's expensive especially for 4. Ports often offer a free shuttle to the gate at least but even a taxi might be economical for you.
7. Entertainment on-board is usually of a high standard but can get repetitive. Some cruise lines now partner with known brands such as Cirque du Soleil or Blue Man Group, sometimes with a fee for the show.
Overall, I enjoy cruises but it is not very economical and I would not cruise with kids. Your base cost will probably exceed €4,000 for a week. Everyone is different though. For a first cruise, I would recommend Royal Caribbean but I use MSC because of price.
Bon Voyage
Slim