Our family did the 3 day self guided walk with Wilsons Abel Tasman first week of March. Our 1 year old and 3 year old came with us in backpacks and Grandma had our lunches in her backpack. We have a good level of fitness and the kids have spent a bit of time in the backpack so are comfortable to spend long periods sitting on our backs.

Day 1 we walked from Marahau to Torrent Bay (16km) and was achievable but a big walk and big day. Our backpacks were approximately 18-20kg. Started walking at 11.30am, stopped at Apple Tree Bay for a swim (1.5hr stop) and we arrived at Torrent Bay lodge at 5.30pm.
Day 2 we thankfully decided to reduce the walking down to 7km and catch a water taxi from Torrent Bay to Tonga Quarry Campsite. We spent 2.5hrs at Onetahuti Bay which was amazing then walked to Meadowbank Homestead at Awaroa Bay. The walk was significantly steeper and harder than Day 1 so we were grateful to only walk 7km.
Day 3 we were unable to do the walk from Awaroa to Totaranui due to a 2hr walking time limit to get there (water taxis at each end of the walk determining the 2hr limit). So instead our guides advised us to take water taxi back to Anchorage and do the loop walk to Te Pukatea Bay which was an easy walk and beautiful beach swim spot.

The accommodation was perfect and catered well for our 1yr olds with cots provided at both Torrent Bay Lodge and Meadowback Homestead. Hot showers, comfy large beds and plenty of towels and toiletries to recover from the big walks.
Cold beer, cider and wine was great at the bar.
The food was delicious with good menu selection and really thoughtful meals specially for the children. The lunches provided were also great for everyone.

Recommendations for people thinking of doing this experience with young children - take LOTS of snacks and allow enough time for plenty of stops. You definitely need a good base level of fitness to do this walk with heavy backpacks on.
Good footwear is critical as there are steep sections of the walk which are slippery due to loose stones on the track. A walking stick would have been helpful to maintain stability going downhill with the extra weight on the back.
Take lots of water - we carried about 4L and never had any left over once we reached camp each night so probably wasn't enough.

The water taxi are really safe and easy to onboard with children - this was not a concern.
There were no carseats or seatbelts in the Wilsons transfer bus so children sat on our laps.

The guides were really helpful and although we did the 'self guided walk' this time we would recommend having a guide for the walk with children next time. The best way to ensure you safely reach your destination in reasonable time.

I booked the 5 day walk and kayak tour, it was a great trip. With this trip it was the best of both worlds. The accommodation, food and staff were fantastic.