This is meant to be a battery cut-off switch. Due to certain design constraints, the low side needs to be switched instead of the high side.
The switching circuit is permanently connected to the battery (3V and BAT_GND). SYS_GND is the GND of the secondary circuit, which is permanently connected only to 3V. The goal is to switch the secondary circuit's GND to preserve battery life.
The secondary circuit's maximum constant current is 20-30mA with the possibility of short spikes <50mA.

INT1 is an interrupt that will be set to be high under certain conditions. The IC has a guaranteed INT high voltage of 0.8xVsup. Battery voltage is 2.0-3.0V, so 1.6V to 2.4V to drive the gate. The transistor used is an AO3400A.
The INT pin is configured as Push-pull, so I do not believe it can ever float. The configuration is such that there can likely never be any frequent switching. There will always be hours, minutes, seconds between switching, never ms or ns.
I have already omitted a gate series resistor, as the secondary circuit should not be able to compromise the switching side and if there is a failure catastrophic enough in the secondary circuit, saving the switch isn't important.
There are no other connections or signals between the two circuits.
Disconnecting ground from the secondary circuit will likely only ever happen when the secondary circuit is idle (very low ±1mA current).

Taking that into consideration, how useful would R1 and C2 be (if at all)? The benefit of removing them would be some space savings and lower BOM. Is the AO3400A a better choice than a 2N7002?

 

I cannot see a reason for R1 & C2.
What ground is common to INT1? Is it common with SYS_GND?
Are you certain you want a N-MOSFET?
More of the schematic will help.

 

What ground is common to INT1? Is it common with SYS_GND?

BAT_GND. This is permanently connected to the MCU from which INT1 originates.

Are you certain you want a N-MOSFET?

Yes, due to how the battery holder is designed in the toy, it is much easier to physically remove the battery negative contact and solder in the switch. It is hard to explain without a picture which I don't have unfortunately.

More of the schematic will help.

INT1 comes from the MCU and there is virtually nothing else in the switch circuit apart from some passives. The "secondary circuit" is a toy PCB for which I don't have a schematic.
The point of the switch is very simple - the same action as if I lifted the battery negative side out of the holder for a few minutes a then put it back. Or jammed a plastic shim into it like toys have at the store.

 

Now I understand.

1.6V to 2.4V to drive the gate.

With a small G to S voltage, you need to look at the curves below, left side. If you had a good solid 3V you cannot get more than 100mA through the MOSFET. You don't have 3V!!

With a small G to S voltage you need to look at the curves below, left side. AO3400A If you had 2V of drive you should be able to get as much as 10A through the MOSFET. I don't know what 1.6V will do.

The IC has a guaranteed INT high voltage of 0.8xVsup.

First find out how this test was done. Probably with some load to ground. I don't have time to chase this down. It might be that the IC will pull up much better with a lighter load. It might be the test was done driving in "TTL load". It might be that a 100k pull up resistor to VCC will bring the output to VCC.
You have two battels. You want ON to be as close to VCC as possible. You want OFF to be as close to 0V as possible. You probably want a resistor to ground or to VCC because the pin is probably "tri-state" at power up. Don't leave the Gate floating at any time.

 

So the AO3400A looks much more suited for the application.
As far as I can tell, during startup the INT pin can be high-z in push-pull config and the datasheet says that it has a bus keeper for valid state when in high-z. I'm not worried about which state as this can only happen if the battery is being replaced/removed and the switch being on or off doesn't really matter. But it seems that the pin cannot float.
INT high/low should be within 20mV of both 3V and 0V respectively.

 

I have decided to keep the 1M resistor, just in case. The addition current when INT1 is high is negligible and it could be removed if necessary.
Thanks for the advice!