Trailer won't be an issue. Most trailers attach to the rear axle, and the bike can withstand lots of load in that location. Remember that bike has to withstand pedaling loads on the chain, which are far greater than loads any trailer will put in there. The maximal load of a trailer will happen when braking hard. Let's say you brake at 0.5 g, and there is 70 kg on the trailer. That's 35 kg of load. On the other hand, a 90 kg person pulling up on rear pedal at 25 kg and pulling up from handlebars at 30 kg using a 50T chainring creates a chain load of 286 kg. If a bike can withstand a chain load of 286 kg, it can withstand a trailer braking load of 35 kg.
About seat post, that's not optimal for a child seat. The child seat will probably have lots of leverage, which can damage the seat post or maybe even the seat tube depending on how long the leverage is and how much weight there is for the child seat. Most seat posts are not made for that kind of leverage.
Far better is a child seat that attaches to the rack attachment points on a bike. For example Surly Nice Rack can carry 36 kg. Presumably the frames that this rack is intended for (Long Haul Trucker, Disc Trucker) can withstand the said 36 kg. So a touring bike frame should be able to carry a lightweight child.
Pick a touring bike frame. It will do anything you want, pull a trailer, carry a child, carry lots of cargo on the rear (and maybe front) racks.
As for e-bikes, a hub motor does not have the capability of detecting how hard you pedal, it's binary (pedaling or not). So the riding feel is very unnatural, in contrast to the natural feeling of mid-drives with proper torque sensors.