Traveling with kids, on most “no frills” airlines, means that you need to budget for more than for the advertised ticket price. Many airlines are adding “extras” into their income by charging people for pre-boarding. At one time this meant anyone who wanted to get on the plane after the passengers flying with children or those who needed assistance with boarding. 
Some “no frills” airlines have made the decision that all passengers are equal and therefore if you are traveling with children (or are mobility impaired) then you either have to wait with the majority of passengers, or pay a supplement per person in your party in order to board the plane in advance of the main boarding call. Although you still have to board with everyone else who has paid for pre-boarding, you should at least have a chance of getting your children safely onto the aircraft and into seats close together well before take-off.
With the cost of refreshments onboard “no frills” flights you should try and take these with you. With restrictions on liquids you may find that some foodstuffs and all drinks are prohibited before security so check with your airport before organizing your on-flight luggage. If you can take some food, perhaps still sealed in original packaging, then this will at least cut some of your costs. Check online if your airline advertisers what snacks it provides and how much they cost – this will allow you to make a decision whether or not it’s cheaper to pay the airport the cost of snacks in the departure area, or if it’s not worth the stress and just plan to pay the airline once you’re on the plane. You might want to carry some candy in your pocket anyway in case you’re seated in the area that will be served last by the snack trolley – it will help kill the hunger pangs in impatient little travelers.
“No frills” cabin crew are not always helpful! If you are negotiating your way up the stairs to the aircraft with two children, a baby and a handful of hand luggage, including diaper bag, don’t anticipate any help from the staff standing at the top of the aircraft stairs. Try to condense as much of your hand luggage as possible into a bag you can secure over your shoulder, have a baby sling to keep the baby safe, and attach any stuffed toys/blankets to your child with a safety pin so that if they fall out of their hands, they won’t drop to the ground. Put your children on the stairs first and block anyone else from getting up behind them except yourself! Tell your children to find the first row of seats and sit in it, you’ll be right behind them. Have your boarding cards and passports if required stuffed into the baby sling behind the baby so they’re easy to reach when you get to the top of the stairs. Remember to replace them there when you get off the plane so they’re ready for passport control at the destination airport.
Once you’re seated, and know what you’re doing, there’s not really much difference between “no frills” and traditional airlines. These flights can save money, but watch the price of the “extras” because if you have a few children you might find that paying half price on a traditional airline that won’t expect you to pay for the privilege of safely boarding your kids onto the plane, and offers free refreshments, is actually cheaper than flying “no frills” with supplements!