Yay, holiday fun! I have an almost 4 year old and 8 month old and we went on holiday recently.
Packing - We have a mammoth packing list that we wrote in Excel in different sections (e.g., DD1 clothes and then underneath things like hat, swimsuit, socks, etc. etc. etc.) We've found having a comprehensive list means we actually pack much more carefully. Think through all the admin bits as well, like GHICs (and a Euro/pound coin for trolleys at the airports!). I always find we need fewer clothes than a regular week at home as they spend so much time in swimming stuff, re-wear stuff more frequently, etc. We find a very small bottle of travel wash and washing up liquid (for washing up water bottles) handy.
Accommodation - not sure what kind of place you're staying, but if it's a child-friendly hotel, then they might have items you can borrow (e.g. pram, etc.) Check the website. We take portable blackout blinds and a plug-in nightlight when we go anywhere.
Plane - lots of snacks! We tend to have one bag that's nappies, spare clothes, etc. that we don't need to go into frequently and then the other is entertainment and food mainly. Colouring, water pens, books, etc. etc. Spare plastic bags are useful.
Have a wonderful holiday!