Can i just say how pleased i am that you have chosen to travel with your family. Where are you off to?
I have travelled with my daughter, now 5 since she was 8 months old. It has been trial and error a lot of the time. I have set up a facebook page and a blog to document some of my travels but mainly to inspire and help other families travel and give them the confidence to do it.
Please do visit my page, i would love you to feedback on your holiday.
I would give the four year old her own backpack. I had a LittleLife backpack for my daughter which has the 'lead' attached. This way they feel as though they are carrying their own toys. I usually take 'three stories' or a book with lots of stories in.
I have found that the wipe clean writing or colouring books are great to take with you. I usually take a small pack of colours and one colouring book. When she was younger lego was good. simple snap cards are also great. I used to take the 'pocket books' on in the night garden or Ben and holly...
As for beach things, bucket, spade etc..you can get these when you're there. A good sarong is a great idea..makes a lovely play mat.
Are you also taking a UV beach tent? These are great for this kids and provide a welcome break from the sun and a great play tent.
When you are there a small blow up boat makes a good buy and a little paddling pool for on the beach or at the appartment.
It depends on where you are going but nappies etc can be bought when out there, no point in clogging up your bags!
On the plane, if it is not long haul then the ipad does work a treat...preload their favourite shows..although ive had to set some boundaries with mine whilst away as lilly wanted it all the time!
I have a lot more tips! visit my page.
Thanks, good luck