My daughter is 5 now. She's been hauled by bike since her pediatrician gave us permission. We've owned most of the child hauling accessories at this point. Here's a brief summary of each.
Thule (Chariot) 2-wheel trailer. Used from 7 months (first as a ski trailer behind fatbike) to 5 years (as a stroller). Bike trailer mode was good for ages 1-5. Reduces cruising speed by 25%. Easy to keep kid comfortable from weather. Too little suspension for dirt roads.
Bellelli Pepe clamp rear seat. Used from 2yr to ~3.5yr. Out of the box, it was tilted too far forward for my daughter to be comfortable. I 3d printed some shims to tilt it back about 8 degrees; then it was much better for her. Suspension adequate for dirt roads and easy trails. Dress her like she's riding a motorcycle. Reduces bike speed by 10%.
Hamax Caress rear seat. Used from 3yr to ~4yr. Could have used from ~18mo. This seat was awesome. Lots of suspension. Easy to adjust the recline angle. Dress her like she's riding a motorcycle. Reduces bike speed by 10%.
Weehoo Turbo single wheel recumbent trailer. Used from 4yr to present. I swapped the rear tire for a 20 x 3.0" tire for some suspension. It's better on dirt roads than the 2-wheel trailers, but not as forgiving as the Pepe or Caress. With sun and rain cover, it offers decent protection from weather. She can pedal when she feels like it. Reduces bike speed by 20%. The Weehoo is surprisingly heavy (~35lb) and awkward to transport by car compared to the other seats and trailers. The 2-seat Weehoo might be a good choice for you.
Strider balance bike. I think I gave this to her around 18mo. She enjoyed it most at 3yr. Added XL seatpost at 4yr. She's completely outgrown it at age 5, but occasionally rides it in our basement. It definitely helped her to learn how to ride a pedal bike without training wheels.
Islabikes Cnoc 16. Used from 4.5yr to present. She got the hang of riding really quickly (maybe 2 hours?). She loves riding her bike (mission accomplished).