If you are going abroad: valid passports, EHICs, currency, insurance (travel, car if driving), international breakdown cover. Tickets.
Whether going abroad or not: medication, tickets, phone chargers, rain coats, sunhats/glasses, sun cream. Jumpers, different sorts of shoes. Little first aid kit (anything liquid in hold luggage if flying). Sewing kit, nail kit. Corkscrew!
Nappies, wipes, cream. Toys. Ipads for the journey plus headphones? Books, colouring. Snacks for journey. Pushchair. Car seat/booster If needed.
Any kitchen bits if self-catering?
If you have done last minute washing, don't forget to pack the dry stuff 

That's all I can think of for now!