I went from Discovery 5 to M3 and get 1 big and often wet golden retriever and two 6ft+ teenagers in the back, no problems at all. Headroom helps. That said I’ll be swapping for the model Y when launched. Dont miss the Disco one bit
I would say that 2 kids and 2 dogs in the back seat of a Model 3 is a stretch too far. However, your dogs are not too big so it would be quite feasible to have 2 kids and 1 dog in the back seat, with the other in the footwell of the front passenger seat. Restraint / safety perhaps an issue but for short and irregular trips I think you’d manage just fine like this.
I regularly have 1 dog, approx 20kg, and 1 kid in the back seat. Use an Orvis hammock, which is great quality, Tried a cheaper version from Amazon that lasted only a few trips before puppy had destroyed it.