Preparing your route:
Take the time to look carefully at the map and its contours or carry out a reconnaissance of the route.~
Take the opportunity to check out the orientation and try to choose shady trails in summer - the whole family can keep cool and it will be more pleasant when you take a break.
Remembering that a child aged 3-4 walks at about 1 to 2 km/h (and takes a lot of breaks), the topoguide estimates will not be of much help to you because the schedules they provide are for an adult walker. Plan for some contingency, so you can enjoy your hike without worrying about the time.
Don't forget to check the local weather and don't worry about postponing your plans if the weather is bad or it's too cold. Children are much more sensitive than us and hiking should remain a pleasant activity for them.~
Routes with refuges will allow you to take a long break and be sheltered.~
Finally, avoid cable cars and gondola lifts, which certainly reduce the effort but can cause decompression problems in young children.