@Voldarian_Empire It won't be a problem, any age can be merged, even infants. I have many times merged children when I was wanting to do something with them but didn't want to play their households after I was done I sent them back home.
This is how or where you need to go to merge a sim with another household. I'll use the Goth family as an example since you mentioned Alexander. :)
There are two ways you can go about this with the same result. Either click on the sim's house they live in, or in the top right corner click on the thing I have the red arrow point to. When you click on the home, look down at the lower right side of the picture, click on the one I have the yellow arrow pointing to. These both do the same thing.
https://scontent-dfw5-2.xx.fbcdn.net/v/t1.0-9/45256758_1142479935902300_9145715944356577280_o.jpg?_nc_cat=100&_nc_ht=scontent-dfw5-2.xx&oh=0c4e8dfa2e487c900d3d7144fa18ac2f&oe=5C7BA231
Next you should have a one for merging sims or households with other households, like this first picture in this group.
https://scontent-dfw5-2.xx.fbcdn.net/v/t1.0-9/45238729_1142478009235826_1977095131362230272_o.jpg?_nc_cat=110&_nc_ht=scontent-dfw5-2.xx&oh=4571e6832b511b36eb0c905e27bb0560&oe=5C466F7F
Choose which household you want to merge with, and click on the sim you want to merge, and afterwards click on the check mark at the bottom.
Now you will have Alexander with the other family of your choosing. Do what you need, and afterwards you can just repeat the earlier steps to put him back with his family.
https://scontent-dfw5-2.xx.fbcdn.net/v/t1.0-9/45221931_1142478242569136_5117304613782945792_o.jpg?_nc_cat=109&_nc_ht=scontent-dfw5-2.xx&oh=0e2dbf2114a989e7f9a18814417d28b8&oe=5C44C8D8