My daughter still goes in a Hamax siesta at nearly 3. It's seatpost mounted and we swap it between bikes. The seat is rated to 22kg (which from the growth charts should take 99% of children up to age 4 and 75% to 5½. We're anticipating being able to use it when she starts school. You don't need to include the weight of the balance bike as that wouldn't be carried on the toddler seat.
The rack-mounted seats don't have a much higher limit - in fact then the limit is usually the rack itself, which has to carry the weight of the seat and the balance bike for you. So you may actually find a lower weight limit overall despite a higher child weight limit.
But on both bikes we have a rack underneath.
I haven't tried carrying her balance bike at the same time yet - she's not yet into going very far on it. But putting the balance bike vertically, with the handlebars at the top rear resting on the rack, and strapping the balance bike frame to the rack should work (at least on 700C/29er etc.).
I managed to get some quick pictures (I didn't even have time to arrange everything somewhere less cluttered so sorry about the quality). I wasn't quite how I envisaged:
Note that the balance bike isn't strapped down at the moment -- it would need to be. A strap attaching the fork/steerer tube of the balance bike to the rack crossbar, and another attaching the downtube to the diagonal strut of the rack against which it's almost resting should be secure. You may want some padding e.g. blocks of foam. I like the fact that the strap wouldn't be taking the weight, just holding it steady. I also didn't have time to test ride it, but heel strike shouldn't be an issue given where my pannier comes to. The rear wheel was probably 15-20cm off the ground, and a bit further out than a pedal - watch for kerbs.
A few notes about my setup that may affect this:
- I've set the seat slightly higher than recommended so I can still use the rack for a pannier.
- The rack is quite narrow (Tortec velocity hybrid) to fit under the seat properly (I had to take my Topeak super tourist DX off as it didn't fit.
One note of caution: This much load this far back affects the handling quite a bit. I built a rack extension when mine was younger and there were times when the front wheel had very little grip. I solved it by shifting weight forwards (D lock on the forks, handlebar bag for all the stuff you have to carry with a child etc.