I'm not necessarily going to answer the best holiday question, as all the trips we've done have been brilliant for different reasons, but thought i could offer some insight to explain how we built up the travelling from a young age!
We did the usual European villa/apartment/AI from 12 months -Majorca, Tenerife, Costa Del Sol, Bulgaria etc.
When DD turned to 5/6, we started city breaks (Rome, Paris, Barcelona, Nice, Edinburgh, Lapland etc) - building up on the sight seeing, stamina for walking, travelling with small luggage volume etc. - and the standard Disney Florida & Paris for longer breaks (much better value then, than now)
When DD turned 7, we moved onto guided tour holidays (we used Explore Family Holidays, but there are plenty of others). Started with India, then Vietnam & Cambodia, then Thailand, before graduating (!) to a self tour of Japan in 2019.
Covid put pay to alot of our travel plans (we had Costa Rica booked for 2020 and Canada booked for 2021) and the uncertainty was too much for me, so I cancelled them all I took a couple of years doing UK hols like the Lake District, Yorkshire Dale's, Pembrokeshire (which are all just as beautiful as the global sights)
We are back to it with city breaks this year - Vienna, Prague, Budapest all booked so far - but a massive Brazil Argentina & Peru trip booked for 2022 in celebration of GCSE's.
The biggest thing I would say, is to work to your children's pace, their tolerance levels, and with lots of flexibility and you'll be just fine.
We loved the guided tours when she was young because all we had to worry about was getting on and off the coach at the right times, the tour guide did all the rest - planning, ticket nying, knowledge, research, journey planning, even where to eat in most days!!!