Stargazer.
Greetings, all of the tag alongs that I've seen connect to the seat post and need ample area above and behind your rear wheel when you turn so they can pivot and follow through. Obviously this negates putting a child seat there. I'm not a fan of child seats in the first place and certainly not one for them in the front. The child's movements up front could spell disaster at the wrong time and has to impair your immediate view of the road as well. At a fun bike ride last year I saw somebody who had a burley buggy and behind that was a tag along. I think they did some custom welding to get that to work but it looked interesting. I'm not sure where you ride but your trailer would start to get pretty long and offer all sorts of new problems.
Here is my .02 Plan a route that your 5 yo can ride on her own bike with you and the 3 yo in the buggy. That route should be enough for the 5yo to get tired but not exhausted lol. When you finish that route (at your home) then leave the 5yo's bike at home put her in the buggy and finish your route with the extra weight.
Ok and if you have gobs of cash lol here another Idea.
Buy a new bike,, not just any bike but a tandem. Your 5yo could ride the back portion and you up front. Then you could still attach the buggy to the back. I'm assuming that you're petite to and that a small frame tandem would work size wise for you and your 5yo. Oh well it's just a thought lol.
Chris