A bit unfair I'd say. No reason to give up on an older dog that has trouble with something. Sassy was retrained many times before we figured out how to communicate what I wanted from her. When we finally got loose leash walking down she was over 5 years old and that last retraining was done along with a younger dog. First train each dog separately then together then train during actual walks first separately then together.
Since training sessions take more time to set up than actually train adding your older dog to the class won't take but 2-3 minutes more each session. In fact you can start training older dog and you will have a better idea what you are doing when you get your puppy. Puppies generally have the attention span of a gnat and it can be extremely difficult to figure out when to click amidst all the extra stuff they are doing.
Take your clicker and treats in a quiet room. Stand with a treat held to your side and when dog comes to see what is in your hand click then give the treat to the dog. Once dog gets this start moving around the room clicking exactly when dog comes to your side. When you cannot lose the dog move to another room then outside where he can be off leash then put the leash on and take it on the road. Dog will get distracted and want to go to the end of the leash. Fine let him. You freeze in place. When dog comes to your side then click and treat and move. I had to stop and start because Bucky would just continue charging to the end of the leash even if I clicked and treated as I was moving. He needs to stop a lot to remember what's going on. You reward what you like - being at your side and ignore what you don't like - tight leash. This means you won't be getting a whole lot of exercise walking the dog for a while. It means you look like an idiot to the neighbors. It also means you won't be so frustrated with the leash pulling because you are seeing what the dog is doing right, walking loose leash for 5 seconds out of every minute!
As for the excited misbehaving when you are being goofy playing with the kids try going through Karen Overall's Relaxation Protocol. It's a down stay mat training that lays it all out for you. While it's a 15 day program you can stop at any point either of you is having trouble, back up to what you had success at and go through that troublesome day tomorrow. Bucky was a sponge for this and we only had a couple sticky spots. Going back to the sticky spot the following day? It wasn't sticky any longer! Amazing.
Since the program requires the dog to at least know how to sit or down and your dog isn't able to down for you try what worked with Bucky. Leash the dog, sit down with a few cookies in your hand and wait. I played a game on my phone. Good thing because it took about 5 minutes for him to lay down! I clicked and tosses a treat away from him so he had to get up to get it and waited for him to lay down again. After several downs I started introducing the cue word as he was actively laying down. Rinse and repeat. I was shocked on the third day when I asked my other dog to lay down to get a treat and he lay down next to her so didn't go back to those leashed sessions. Since you don't have a helpful dog to show the ropes it probably will take a few more sessions. Once he's got it move to other places in the house, drop the leash, take it on the road. Count on starting over again for about 10 new places as dogs don't generalize well. Once he's got that down means down in the kitchen, bedroom, patio, driveway, sidewalk here and there and so on he knows down is down so long as things aren't too weird which is where Karen Overall's method picks up.
