The OP has an option of getting 2 contiguous seats guaranteed without an additional charge. This is because with a child that young, she has the option of using her carseat on board.
FAA regulations mandate that a
ticketed child in a carseat MUST be seated next to one accompanying adult, and in a legal seat position (usually a window seat, & there are some rows where carseats cannot go). This is per emergency evac rules; you cannot have a child restrained in a carseat without his/her adult seated next to it, because the child cannot free himself from the carseat in the event of an emergency evacuation.
It doesn't matter if the airline
wants to charge to guarantee it; they can't require you to pay to get it. Use of an aircraft-certified carseat must be allowed at no extra charge if the child has a paid ticket, if there are any no-charge seat options sold on the flight at all. For courtesy and to makes life easier for everyone involved, it is only polite to call after buying to get the seats flagged, but if the seats are not together on day of flight, the FA will make it happen somehow. (This is one of the very few situations that empowers an FA to force someone to switch seats, though they will always try to solicit a volunteer before resorting to that.) In practice, this safety rule normally requires that you put the child in the window position and the adult in the middle seat.
Note that this only works if the carseat is used in flight; you can't just call ahead and say that you will be using one, and then show up without it; the airline has every right to yank the better seat assignment out from under you if you tried that. In addition, note that this only applies to aircraft-certified carseats with a 5-point harness in use. (Certified seats have a sticker on the side or back of the outer shell that notes certification for aircraft use.; details of the wording can be found in the linked regulation below.)
The rule only applies on US-based carriers; it is Federal law.
The code section is 14 CFR 121.311 (
https://gov.ecfr.io/cgi-bin/text-id...true&node=pt14.3.121&rgn=div5#se14.3.121_1311)